Hello Sunwarrior and welcome to the GM-chair.
I suggest you ask yourself a question:
Do you want the encounter to challenge your players? If yes, then tailor the encounter to be challenging. If not, then make the encounter so its fitting to the world you're playing in.
I'll assume you want it to be challenging, and so we come to another question:
Do you want the infernal bloodrager to be a single boss that challenge the party in 1v4 combat, or are you fine with him having backup?
Depending on which you prefer, the way you create your encounter will change. However, no matter which you prefer, we can look to the rules for help on the basics of how to design the encounter.
If this fight will be against 4 level 2 player characters with standard 15-20 point-buy abilities, then the exp budget for your encounter would look like this:
If the encounter is supposed to be average (which means the challenge rating of the encounter is equal to the average party level) you have an exp budget of 600.
If the encounter is supposed to be challenging (CR = APL+1) you have an exp budget of 800.
A Hard encounter (CR = APL +2) would have 1200 exp.
And finally an Epic encounter (CR = APL +3) would have 1600 exp for its budget.
Depending on your decisions on whether you want the boss to fight solo or with minions, and the decision on the difficulty of the encounter, you build it differently.
If you'll tell me what you prefer, I'll see about helping you build something :)
-Nearyn
My pleasure =]
Here's a sample character I've made for the infernal bloodrager. I've made a challenging version - if you want a stronger or weaker version, lemme know.
infernal bloodrager - CR 3
exp 800
Male human bloodrager 4
CE Medium humanoid(human)
Init +2; Senses perception +7
DEFENSES
AC 20, touch 13, flat-footed 17 (+7 armor, +2 dex, +1 dodge)
hp 46 (4d10+12con+4FC+4Toughness)
Fort +6(+2 vs poison), Ref +3, Will +3
Defensive Abilities Uncanny Dodge; resist fire 5;
OFFENSE
Speed 40ft.
Melee mwk glaive +6 (1d10+3/x3)
Ranged javelin +5 (1d6+2/x2)
Special Attacks Hellfire Strike(Su) 3/day
Bloodrager Spells Known (CL 4)
1st(2/day) - enlarge person, shield
STATISTICS
Str 14, Dex 14, Con 16, Int 8, Wis 10, Cha 12
Base Atk +3; CMB +5; CMD 17
Feats Toughness, Dodge, Iron Will
Skills acrobatics +8, intimidate +8, knowledge(arcana) +4, linguistics +0(1 rank for abyssal), perception +7, spellcraft +4; ACP -4
Languages common, abyssal
SQ Bloodline(infernal), bloodline power (hellfire strike), bloodrage (13 rounds/day), uncanny dodge, blood sanctuary, blood casting, eschew materials
Combat Gear potion of cure moderate wounds, potion of bear's endurance, potion of infernal healing, oil of magic weapon; Other Gear +1 chainmail, masterwork glaive, javelin x5, dagger, 5pp 35gp
treasure 550gps worth of treasure
How he plays: If he knows the PCs are on the doorstep he casts shield which last 4 minutes, and downs his potion of bear's endurance which lasts 3 minutes. When he notices the PCs he applies his oil of magic weapon and casts enlarge person on himself before raging and entering melee.
When enlarged, in a bloodrage and under the effects of Shield and Bear's Endurance his statblock looks like this:
infernal bloodrager - CR 3
exp 800
Male human bloodrager 4
CE Medium humanoid(human)
Init +1; Senses perception +7
DEFENSES
AC 22, touch 11, flat-footed 20 (+7 armor,+4 shield, +1 dex, +1 dodge, -1 size)
hp 62 (4d10+28con+4FC+4Toughness); fast healing 1
Fort +10(+2 vs poison), Ref +2, Will +5
Defensive Abilities Uncanny Dodge; resist fire 5;
OFFENSE
Speed 40ft.
Melee mwk glaive +7 (2d8+8/x3)
Ranged javelin +4 (1d6+5/x2)
Special Attacks Hellfire Strike(Su) 3/day
Bloodrager Spells Known (CL 4)
1st(2/day) - enlarge person, shield
STATISTICS
Str 20, Dex 12, Con 24, Int 8, Wis 10, Cha 12
Base Atk +3; CMB +9; CMD 20
Feats Toughness, Dodge, Iron Will
Skills acrobatics +8, intimidate +8, knowledge(arcana) +4, linguistics +0(1 rank for abyssal), perception +7, spellcraft +4; ACP -4
Languages common, abyssal
SQ Bloodline(infernal), bloodline power (hellfire strike), bloodrage (13 rounds/day), uncanny dodge, blood sanctuary, blood casting, eschew materials
Combat Gear potion of cure moderate wounds, potion of bear's endurance, potion of infernal healing, oil of magic weapon; Other Gear +1 chainmail, masterwork glaive, javelin x5, dagger, 5pp 35gp
treasure 550gps worth of treasure
Now he hits pretty hard, his glaive reaches out 20 ft., and he can spend a swift action 3 times during the battle to add 1d6 fire damage to his attacks.
Hope you can use it, or that if you can't, that you'll have found some inspiration in it. As I said, if you need him taken up or down a peg, I may be able to help with that also, just post it here and I'll help if I don't forget to look :P
Have fun.
-Nearyn
EDIT: Fixed his HP, I had gotten it in my mind that he was level 3 for some reason.
I really can't recommend enough using this guide. It has helped me immensely in understanding the best way to craft encounters for the groups I run.
Glad to hear you like it.
The way you calculate exp is on a fixed track depending on the strength of what you want to run. You can see the table HERE
It's the table called Table: Experience Point Awards
This table shows you how much exp an encounter of a certain challenge rating is worth. So let's say you want to design a CR 11 encounter. You just take a look at the table, find CR 11 and see that it awards you a budget of 19.200 experience points. The table further shows how you can split these points between party-members in case you have a party with 1-3 PCs, 4-5 PCs or 6+ PCs, that's not really important to what we're doing, but it's a helpful addition.
Okay so our CR 11 encounter has 19.200 experience points. Well, from here on we can basically do whatever we want. All monsters, NPCs, Traps, Hazards and what have you, has a challenge rating. Monsters and prepublished NPCs will usually have their CR stated up front.
A juvenile Green Dragon is CR 9 and worth 9.600 experience points. So if we wanted to, we could have the party attacked by 2 juvenile green dragons, and we'd have spent our 19.200 experience budget, meaning these two creatures are a CR 11 encounter. There'll be times where you have to eyeball it a bit. For example no matter how you look at it, there is no way a group of 12136 lvl 1 commoners will ever constitute a CR 25 encounter, even though the exp calculation says they do - they'll get wiped out by a single level 9+ wizard. Learning how exactly to judge these things can be hard, but reading guides can help, and its basically something you'll learn the more you try it.
Now the question then becomes, how do we calculate the CR of creatures that do not have it written down in a book? Like when we make a custom bloodrager like the one I did? Well, there are rules for custom monster creation in the bestiary, those can be followed with high hopes of succes, but when creating NPCs it's really easy to do.
If you're making an NPC with levels in an NPC class (commoner, expert, aristocrat, adept or warrior) and give him gear suitable to his level (as noted in the NPC creation tables) his CR will be equal to his HD minus 2. You detract 1 for it being an NPC class (they're weaker than the heroic classes), and 1 for having very little gold to spend on gear.
If an NPC has levels in a heroic class, but you still build him with the suggested wealth (as is the case for the bloodrager I gave you), the CR is equal to his HD minus 1. This time you don't detract 1 for the NPC class, since his class is heroic and he's stronger for it. You still detract the 1 for less GP to use on gear, though. If we had given him gear with a value equal to that of a normal player character of his level (level 4 heroic NPC gets 2400gp, level 4 heroic PC gets 6000gp) then we don't detract 1 from the CR. So if I'd made the bloodrager with 6k worth of gear, he'd be CR 4, instead of 3, and then he'd be a Hard encounter, rather than a challenging encounter.
There are also rules for making combinations of NPC classes and heroic classes - you can find them in the relevant rules chapters :)
You'll also note that normal NPC-classed NPCs are usually constructed with point-buy 3, where heroic NPCs are constructed with point-buy 15.
But these are just minor details when constructing and since we're interested in how we calculate the experience budget, the only thing you really need to remember is the rule that: If it's NPC-classed detract 1 from the CR. If it's has less gp for gear than a player character of the same level detract 1 from CR.
With this knowledge you can make any combination of stuff you want.
So let's make a final example. You have a group of 4 level 2 PCs. Their APL is 2. You want the encounter to be hard, so you want it to be CR=APL+2 or CR 4 in this case. We check the Experience Point Awards table and see we have 1200 experience points to spend.
So we could make this bloodrager dude we just made, who costs 800 exp, and then we have 400 exp left for other stuff. We could give him a bodyguard who was a level 3 warrior (CR 1 = 400 exp) or we could have there be a CR 1 pit trap in the room where the fight is going to be. So as long as we keep track of the experience we have left to use, and what we use it for, we can tinker around with any combination of challenging stuff to spice up the encounter, and give our players a fun and memorable time :)
Hope it helps.
-Nearyn
You have been given great advice of a mechanics nature. I would go with what has been already mentioned.
My suggestion to you is more in the "theatre of the mind" department. Your big bad is going to be a challenge, but without some description, he risks being just a dry stat-block.
Decide on a first impression for the bloodrager. Greasy black hair, or a mangled ear, or a bulbous nose, or missing teeth for example. Then decide on a mannerism for the NPC. He blinks a lot, gesticulates wildly, licks his lips or some other action that you can adopt when you speak as the NPC without being a professional actor. Just one first impression and one mannerism and the NPC is suddenly more than just numbers.
Speaking is a free action, make sure to use it. You don't have to monologue for minutes on end (that will just slow combat to a crawl and annoy your players), just a few words or gestures and you will find players suddenly want to role play out their character's response! Get the players use to interaction and you will be very glad that you did when they meet that NPC that you really don't want them to murder on sight.
Then come up with a list of random names, first impressions and mannerisms that you can use if required. You are wise to keep such a list handy in the event that the players decide they want to talk before they hack and slash. Then when you use one of the names with a first impression and mannerism, jot it down somewhere so that you can refer to it later.
The numbers are important and Nearyn has covered that well. Adding some flavour description to the numbers will facilitate engagement and immersion in the fantasy world your group is creating.
That sounds awesome! Well done.