Hello there Pathfinder community!
This will be my first contribution and first monster created for Pathfinder. I currently DM a group of 6 and began "cold turkey", so to speak, a few months ago. I have no previous tabletop experience, so any help is appreciated!
I gave the Zombie Horde (small) a +1 to initiative over the base initiative of zero - for a single zombie. Since they're working together, it should give them a slight edge. Same thing applies for perception.
AC was increased from 12 to 14, since attacks against the zombies are being dispersed amongst the group. HP was changed from 12 (standard single zombie) to 30 to represent three zombies.
Will save was increased from 3 to 5. I feel that tripling it to 9 would be too overpowered for this monster, even though there are three zombies. I simply added 1 extra will save per zombie.
The "Slam" attack is the standard zombie move, unedited. I added "Swarm" since it's a classic undead assault. The "Consume" attack is only added for the large horde.
Base statistics were increased slightly. Attributes (Str, Dex, etc) were increased by 1 per zombie. Base attack, CMB and CMD were increased to due the horde. It's harder to grab onto a cluster of zombies. I gave the Zombie Horde "Combat Reflexes" since they're similar to a swarm. Passing by it should be met with several outreaching hands/claws attempting to stop the player.
The unique quality of the Frightful Presence ability should be both interesting and not too overpowered.
Human Zombie Horde (small) - CR 3
Medium undead (3 zombies)
Init: +1 Senses: darkvision 60 ft; Perception: +1
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DEFENSES:
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AC: 14, touch 10, flat-footed 14 (+4 natural)
HP: 30
Fort: +0, Reflex: +0, Will: +5
DR/Slashing 5; Immune: Undead Traits
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OFFENSES:
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Spd: 30 ft
Melee: Slam +4 (1d6+4), Swarm +6 (2d6 plus grab)
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STATISTICS:
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Str: 19, Dex: 12, Con: -, Int: -, Wis: 12, Cha: 12
Base atk: +2; CMB: +6; CMD: 16
Feats: Toughness, Combat Reflexes
Special Qualities: Staggered, Frightful Presence*
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ECOLOGY:
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Environment: any
Organization: any
Treasure: none
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Human Zombie Horde (large) - CR 6
Medium undead (6 zombies)
Init: +2 Senses: darkvision 60 ft; Perception: +2
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DEFENSES:
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AC: 16, touch 10, flat-footed 16 (+6 natural)
HP: 60
Fort: +0, Reflex: +0, Will: +7
DR/Slashing 5; Immune: Undead Traits
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OFFENSES:
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Spd: 30 ft
Melee: Slam +6 (1d6+6), Swarm +8 (2d6+4 plus grab), Consume
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STATISTICS:
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Str: 21, Dex: 15, Con: -, Int: -, Wis: 15, Cha: 15
Base atk: +3; CMB: +8; CMD: 18
Feats: Toughness, Combat Reflexes
Special Qualities: Staggered, Frightful Presence*
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ECOLOGY:
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Environment: any
Organization: any
Treasure: none
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Consume: A large zombie horde can begin consuming any creature it swarms over. Against helpless or nauseated targets, a zombie horde deals 6d6 of damage.
*If the total number of zombies are equal to (party size x 3), force the party to roll a Will save from Frightful Presence. A standard party of 4 would need to roll for this save if there are 12 zombies or more (2 large hordes or 4 small hordes). A party of 6 would need to roll for this save if there are 18 zombies or more.
A Zombie horde can occupy a single space or multiple spaces, at the DM's discretion. However, the horde is still treated as a single target.
Bumping once to see if there is any interest.
Welcome to the boards.
With these monster stats, it looks like you're introducing a couple of completely new mechanics to the game. I'd carefully consider whether or not you really want to do that in your game, especially as you say you're new to both to PF and to table-top RPGs as a whole.
First, your concept of a "horde" seems to be a handful of otherwise size Medium monsters that are acting as a single creature. This concept seems problematic with respect to the existing rules set. Does that mean if a PC targets any one of them with a spell that affects a number of individual creatures, they are all affected equally? If you hit one with a ton of damage, do all of them fall? Can one horde go after three different PCs that are in nonadjacent squares?
Additionally, it doesn't make sense for them to have both the Staggered limitation and Combat Reflexes. I'd ditch the Staggered quality, and let them take a move plus a standard action each round, like a normal creature.
It looks like you're trying to base the concept of a zombie horde on the swarm creature subtype. In the case of a swarm, it's a countless number of tiny, diminuitive, or fine creatures that are acting as one. It's hard to apply the same concept to a group of three or six seemingly distinct individuals.
I'd go back to the core concept of what you want and re-work the monster as an individual creature that becomes extra-deadly when cooperating with others of its kind.
First of all, I'd base their stats on that of the normal zombie template. Keeping with your concept above, I'd remove the Staggered quality, and add Combat Reflexes and Toughness.
Now-- what makes your horde zombies special? Give them new or existing special abilities that make them special.
*Acting as One:*
Maybe all horde zombies in a given area are aware of everything that any one of them is aware of: if one is not surprised, none are surprised; if one can see a concealed opponent, they all can. Give them a new supernatural ability:
Communal Senses (Su): Any horde zombie is aware of the sights, sounds, and smells of every other horde zombie within 30 feet. If any individual horde zombie succeeds on a perception check (e.g. to detect a hidden opponent), all others within 30 feet are considered to have made the check as well.
*Swarming a single opponent*
Another concept that you have is that these horde zombies seem to fixate on attacking a single creature as a group, and try to "swarm" the opponent. Normally, a medium or small character can only face up to eight medium or small opponents (one in each adjacent square). It sounds like you want them to be able to attack "through" the squares of other zombies. Give them an ability that does that:
Swarming Reach(Ex): A horde zombie's reach can extend an additional 5 feet through a square occupied by another horde zombie.
And, as for your frightful presence ability:
Frightful Presence (Ex): (30 ft, DC [As Universal Monster Rules]). The DC to resist increases by 1 for every 3 horde zombies are visible. A creature who succeeds on a saving throw is immune to the frightful presence of any horde zombie that was visible at the time the saving throw was made for 24 hours.
Finally, under "Tactics," you can state that horde zombies normally all attack a single opponent until it's down, unless prevented from doing so by physical barriers (e.g. a wall of ice. You will need to determine whether they will mindless walk through dangerous terrain or obstacles to get there. (i.e., will they walk through spike stones? A wall of fire? You make the call-- it's your concept!
I'd probably say that these additions to the zombie template would add a +1 to the CR of an individual zombie. They're most deadly as a group, though, so you'd want to deploy them in mobs.
The above stats I just thought up on the spot, so they may need refining if you plan to use them.
But, back to the core, I would really suggest against introducing new mechanics or replacing existing rules until after you've played the game for a couple of years. Get familiar with the existing rules set-- they're complex enough on their own! -- before tinkering too much.
Good luck!
Thank you for reading my post and offering helpful critique. I read all of it and I understand what you're saying.
I have lots of ideas for Pathfinder monsters... but putting them "on paper", so to speak, is the difficult part. You're right about adding game mechanics, too. I'll try harder to fit it in with the existing game rules.