That’s kinda interesting! I’m curious what will make destroying the phylactery challenging; if players can just walk up and whack it once to destroy it then the resurrection mechanic won’t really come into play.
Besides this, see if you can think of other abilities and mechanics that interact with this resurrection ability and the phylactery. What kind of strategies does the phylactery make players employ, and how do the other abilities help or hinder these strategies?
Alrighty then. I may have forgot to mention that phylactery is an entity with health.
Here are some small concepts, which you may pick and choose from like Halloween candy.
Idea 1: Defensive Spells: The phylactery is enchanted. It will attempt to (slowly) move away from the player and shoot projectiles at them.
Idea 2: Hey, That's Mine!: This concept relies heavily on the fact that you don't destroy the phylactery. The less health the phylactery has, the faster and stronger the Withered Emissary becomes. Still loses health upon resurrection, though.
Idea 3: A Soul a Day...: Whenever the Emissary kills any non-undead mob or player, it will regenerate. It could even play a cool animation where the mob's "soul" becomes absorbed by the phylactery.
These next two ideas don't really have anything to do with the phylactery, but they're cool nonetheless.
Idea 4: Minions Assemble!: The Emissary, being a lich, has undead minions! One of them is the Death Knight, which is basically an armored Wither Skeleton that's been hopped up on soul magic.
Idea 5: I, Monster: Lichdom is a choice. A choice the player could make. Maybe the player could craft a phylactery, use it to absorb the souls of the living, and gain the power of undeath.
There's also something about demiliches, but I think I've covered everything.