I recently ran "Song of the Sky" from the Oracle of War series. This features an Emerald Claw necromancer named Irullan Karnach as the main baddie. In the Oracle of War series, she has a magic item called the Crystal Skull that enhances the attuned wielder's undead-related spells and also allows them to cast Raise Dead 1/week.
I am pondering making Karnach a recurring baddie. Perhaps she has a minion attune to the skull so they can cast Raise Dead on her if she dies. OR maybe it works more like a lich's phylactery, in that it casts Raise Dead on the attuned wielder automatically with the process taking 1 week. However, maybe it slowly and secretly turns the attuned wielder undead!
The PCs killed Karnach when they confronted her in "Song of the Sky". I'm not sure what they did with her body, but I want her to return as a recurring antagonist since I am going to have the Emerald Claw be the main baddies leading into Vol: Eve of Ruin.
I am running an upscaled conversion of the 3.5e Eberron intro adventure, "The Forgotten Forge", at the moment. I made Glaive, the warforged from Eve of Ruin's Mournland chapter be the assassin at the start of the adventure, and I've been considering having Karnach show up at the end. But then I thought maybe that's too soon, so I've replaced her with a Bone Knight. But maybe she could be there with the Bone Knight, monologue for a bit, then leave? Just to show the PCs that she's still alive and is playing a long game? (She doesn't really care if the PCs defeat her minions here, because she can still claim the macguffin they're retrieving later.)
I'm going to be running Grasp of the Emerald Claw soon. I can either replace Garrow (this adventure's Emerald Claw baddie) with Karnach, or I could have a scene where the PCs see Garrow conversing with Karnach (Star Wars hologram style maybe) so they know she's involved somehow.
Eyes of the Lich Queen features a recurring Emerald Claw baddie named Dura ir'Matellan. IIRC she's a Blood of Vol cleric. The PCs can kill her several times, and she keeps coming back, with her final appearance being as an undead creature. I suppose it might be a bit silly to have two recurring Emerald Claw baddies who keep coming back and ultimately become undead at the end, so I could either replace Dura with Karnach or just have Dura be the initial Emerald Claw baddie and then have Karnach take her place later in the adventure.
And then, when I get to Vol: Eve of Ruin, instead of having Kaius take Kas' place in the adventure, I can have Karnach be the secret spy. She wants to usurp Vol's ritual for herself. I'll have the Wizards Three be Chamber agents. Narcy of Xandrar (secretly a gold dragon) from "The Last Word" can stand in for Alustriel Silverhand, while Sur'kil (secretly a silver dragon) from Eyes of the Lich Queen can stand in for Mordenkainen. Karnach, now a lich and masquerading as someone else, can be the secret traitor and the Tasha stand-in. Just gotta figure out who she's pretending to be.