that said evil often more than not is self serving too.
AS: Assasinate Lord Neverember and put evidence of deed on someone else mabe the chief orc.
Ashmadai: corupt Lord Neverember and bring him into the fold or kill him and place blame on the netherese.
Keep in mind if these player characters have histories or a criminal record. Evil players tend to make enemies and may have warrants or bounties. Maybe they owe some organization money back home or unpaid guild fees. Maybe they stole some item and a wizard has offered a reward for its return.
Be prepared to have common folk murdered or anyone for that matter it can happen and there is nothing quiet like this to also make good NPC's jump to bring in or punish these villianous anti-heros.
Consider their motivations, if one or two of this party may be spys or has someone paid them to find something.
Ashmadai are always brushing up on Rituals related to Devils so the interest of Hell could be involved and they could be looking for a Few Evil Men. I actually involved a party in a few encounters with Ashmadai where they had to rescue a member of the party who had missed a few games and was about to be sacrificed in a Ritual that required Impilturian Goliath blood to Summon a Devil. The other was another Ritual Sacrifice which was to summon a Pit Fiend it failed because the party need the crew member of their ship but not before the angered half Summoned Pit Fiend unleased his minions upon the scene of the Ritual. Maybe they want to feed the Dread Ring...
Don't forget Evernight, Shadovar, Thayan, Ashmadai, and Abolethic Sovereignty tensions.
Abolethic Sovereignity, well this party in my game brushed upon them several times and never realised who these NPC represented accept one which the player fled when he realised the person he was following through some caves stopped to stand idle before a stagnant smelling pool where some aberrant fish like creature began to attack his mind. Having a low Will, coupled with my open low attack Roll that nearly missed sent him fleeing for his sanity. Personally I find them nasty and creepy HP Lovecraftian like referenced stories where most any common folk could be enmeshed in a dastardly plot to bring about some horror from the Far Realm. I once had a fishing village covertly overrun by fishfolk who were still converting the races there into fish folk. This party though not evil ended up destroying the whole village out of fear and horror. Mine was near the Lizard Marsh long ago but yours could be near or in the Mere of Dead Men.
Lastly I will say be prepared for greed and maybe sowing that seed may be the weakness of this party. In 1st edition we had a couple of games where one or two evil greedy player killed or tried to kill another player. This happened once when the party came in contact with a cursed magic item and later when the party had defeated some guardian to recovere the lost treasure they were seeking.
It still is often strange when it happens as a DM to remain Neutral in such a challenge because some players get real sensative when stabbed in the back by their friends or party members. Remind them it is only a Game!
Also keep an eye on Arcane Players who use Area effects, that might anger other players. Try to keep a cheerful atmosphere while they destroy each other. Another tactic while they are fighting each other have prepared Cultist, Slavers, a hungry Dragon or Paladins that might arrive to challenge the Party and let them truly determine who is Friend or Foe.
Don't forget the poisoned food or drink, that necromancer who raises his fallen party member as a guardian for the rest of the encounter, the party that decides they can capture some commoners to sell them as slaves to get magic items or a map. Most of all make sure they are prepared to have fun and not leave the game not having had a good time with their evil adventure.
steal magical items to disenchant