What books do you have access to? Some changes might have to be made to get the exact feel you want; there's a lot of material related to this sort of thing in 3.5 books.
The Advanced Player's Guide has rules for the antipaladin. Alternately, consider countering the paladin with an eldritch knight devoted to Asmodeous and a focus on summoning evil outsiders. The most memorable "counter-parties" aren't just mirror opposites - they're unique NPCs with similar but distinct abilities.
One thing that's worth mentioning is that when you've got a lich in your campaign, hiding the phylactery is an art. The bestiary tells you that liches tend to keep it on their person at all times, but that entirely defeats the purpose of the phylactery. A good lich will hide his phylactery far, far away and ward it with ample protection. There are numerous ways to do this creatively. My personal favorite was to have a lich take his phylactery into the middle of a featureless desert and cast "imprisonment" on it, though that can be tough to find. Other measures I've used have been to create an alternate of the Disciple of Ashardalon class (from the 3.5 Draconomicon) in which a dragon is using the phylactery as it's heart.
Try to think outside the box with your phylactery. Create decoys. Don't let yourself think that it always has to be a tiny bauble. I've seen golems and even fortified castles serve this purpose. If you want to give your players a real challenge, hide it on a different plane of existence.
The unaligned cleric option sounds better than the antipaladin (despite facing off against a paladin); clerics are full, armored casters. This is what I would do:
In the Pathfinder Adventure Path: Bastards of Erebus, there are at least 98 optional tiefling abilities; one of which is "healed by positive and negative energy". Use a tiefling, swap Darkness for Healed by Positive and Negative Energy, apply the Skeletal Champion template, and level that nasty surprise up. Many would say that the Undead type neutralizes the tiefling's healing ability based on "Undead" overriding "Outsider (Native)", but the template never implies that the original creature loses its natural abilities; so I'd keep it. Also, change around the tiefling's racial ability modifiers (+2 DEX, +2 INT, -2 CHA) to suit the build.
Yes, but in the context of most game worlds, it just makes sense to have an evil cleric at your disposal if you're fielding large numbers of undead. They channel negative energy so they can heal the troops on a large scale, and that same energy can be put to devastating use against PCs in an all-undead encounter. Plus, they offer one of the quickest routes to bolstering your numbers with their early access to Animate Dead and the ability to utilize the Command Undead feat.
Which roles do you need to fill in this manner?
That sounds like a good idea, I've played combat clerics before, so that shouldn't be too much of a stretch to build. What domains would you suggest? I was thinking something along the line of Death and maybe war or Evil--not sure which one of the last two would work better as both kind of have their ups and downs.The unaligned cleric option sounds better than the antipaladin (despite facing off against a paladin); clerics are full, armored casters. This is what I would do:
In the Pathfinder Adventure Path: Bastards of Erebus, there are at least 98 optional tiefling abilities; one of which is "healed by positive and negative energy". Use a tiefling, swap Darkness for Healed by Positive and Negative Energy, apply the Skeletal Champion template, and level that nasty surprise up. Many would say that the Undead type neutralizes the tiefling's healing ability based on "Undead" overriding "Outsider (Native)", but the template never implies that the original creature loses its natural abilities; so I'd keep it. Also, change around the tiefling's racial ability modifiers (+2 DEX, +2 INT, -2 CHA) to suit the build.
If you want the iconic evil priest, Death and Evil are good partners; if you'd like a persistent foe, use Death and Trickery (Copycat will really make players think about future encounters). I'd build the priest defensively (one-handed weapon and shield), while including a "bodyguard" using a scythe specializing in tripping characters. These guys should use every dirty trick in the book.
I mention the bodyguard specializing in crippling enemies, because these NPCs strike me as a sort of murder-squad: find Iomedean priests, isolate, disable, and eliminate. With this in mind, they (the undead) would likely use methods (in addition to tripping) that handicap the prey and allow for quick kills.
In a game heavy with divine spellcasters, I'd recommend having the bad guys keep a contingent of killers with the assassin prestige class, since they get abilities that circumvent the resurrection of their targets.
Two other questions now, because I can't seem to find it anywhere, what level of control would a lich have over his undead minions? I guess what I mean to ask is, would the undead minions have any will of their own?
The second question is an extinsion of the first. For the trickster clerics I was thinking of a chaotic evil alignment and more along the lines of neutral evil or lawful evil for the other enemy armies. I asked about free will first because a lot of times when I use a chaotic evil enemy, he's just that...completely unpredictable, if things aren't going well he starts slaughtering his own men, or uses his men to make some very messy object lessons to the rest.
A lich would have the same level of control that a human necromancer might have (Command Undead feat) unless he's got some sort of artifact compelling undead subordinance. Make the lich powerful enough to discourage rebellion, but considerate (to his minions) enough to foster cooperation. As for free will, the intelligent undead should have plenty of it; this should be shown in the ways they achieve their goals.
A NE undead priest can be unpredictable as well. Bury controlled skeletons/zombies in an area he/she has secured (waiting for new orders) in order to surprise intruders; with the stage set, the priest could force a captured (or allied) bard to provide music to dance to. Allow the heroes an easy approach, face the priest away from the interlopers (features obscured by cloak), then spin around and surprise them with a hidden army accompanied by music. If the musician stops for whatever reason have the priest ignore everything else until he/she's "convinced" the performer to continue. Wacky things like this make for a memorable villain.
It just occured to me that it might be a nice twist to make a prominent priest a human that's been coaxed to into the services of evil by promises of the secrets of lichdom.
Instadeath alert!!!!!
Unless you roll in secret and sometimes fudge rolls as a DM, I would not recommend having an NPC warrior use a Scythe. If you crit....you'll one shot a PC if you've built the scythe-wielder solidly.
Just a public service announcement.
If you have access/are allowed to use the current Magus Playtest, a Magus makes a very, very beastly NPC melee/magic warrior. I even posted a level 5 Skeletal champion Magus BBEG style build. They have a lot of potential as NPCs due to their ability to somewhat offset the action economy disadvantage NPCs are often up against.
Where would I get my hands on the magus playtest? Is it a free pdf on this site like they did with the finalized new classes from the apg?Yeah, same procedure this time around.
One thing you can do with the Lich's phylactery is go the Harry Potter route and have it broken down into several pieces that all have to be destroyed.
Have the Lich create his phylactery from one of those puzzle sculptures, then break it down and hide the pieces. The PCs have to find the pieces and assemble them before they can destroy it.
Of course, this also means that the Lich cannot reform or be reborn unless the puzzle is put together, but those are the breaks.
This would be especially appropriate if the demon lord has something to do with mysteries, or puzzles.
Here's something I tried once. The lich's phylactery is a wooden sculpture that creates a perfect likeness of a human child. Once the creation is complete, he hits it with "Polymorph Any Object," followed by "Permanency." Bam! Now is phylactery is a living human infant. I admit, the fact that it now grows old and dies makes very little sense from a practical standpoint, but if you can slide that past your PCs, it makes for a fantastic moral conundrum.