I find in the past what makes villains great is what I call the Villain Platinum Package.
1. Power- I think you have that down. This druid sounds scary.
2. Toys- Having powerful items nobody else has. Like artifacts.
3. Base of operations- A well planned lair, with traps, warning systems, magical defenses. For high lvl parties, I prefer lairs to be spacey. Don’t divide it up by rooms, but by buildings. You don’t have to hand draw up every building, just write down the purpose, defenses, treasure, how the building will react to the party. How will the building warn other buildings?
4. Minions- Every super villain has minions. Not just pointless cannon fodder, but some mildly powerful opponents. Cohorts are good. Maybe a druid understudy. This guy is an undead druid, this step should be easy.
5. Allies- Everybody has allies. (How many allies does your pcs have?) Maybe the druid has some powerful fiends giving him advice. Maybe he use to adventure in his younger days, so does he have a party of his own. I know this is the last encounter, so maybe go a little light on this one.
6. Play your villain smart- He didn’t get this powerful by being stupid. He knows he has powerful enemies, and he knows about magic. Even nondruid magic. Have the entire place be rigged with magically defenses, from wizard to cleric magic. (Remember when I was talking about allies.) Spells I like are Forbiddance, unhollow, desecrate, wall of force, all walls, mage’s private sanctum, etr.
Further he should have several “What if” plans ready to go. Maybe he already knows the party is after him. And always have an exit plan.
Take time to review the spells in your source books. Look at every spell and ask “how can the players use this spell to mess everything up?” Be prepared.
7. The edge- He should have some kind of ace in the hole. Something that allows him to do something a little outside the box. Don’t break the rules, but bend them.
EX: the druid spell “Awaken”. This spell is awesome, because it gives trees and animals a voice and mind. However the game creators put a material cost of 2,000 gold worth of herbs and oils. This way nobody can go nuts an “Awaken” the entire forest. However what if your druid learned how to mass produced these herbs and oils. What a nightmare. The players would never see it coming. Also this could bring up moral dilemmas as well. Now the forest is not made up of plants and animals, but people. Also what if some of the “awakens” don’t agree with what the druid is trying to do?
8. The Encounter- If you do things correctly, he should know the party is here, and looking for him. This will give him time to prepare. Maybe grab some of his minions, and go to the party. The Five things that matter in encounters are…Who has surprise, who has the terrain advantage, How prepared both sides are, minions and how powerful the villain is.
9. Be evil- Being evil is easy, and being good is hard. Always has been. Don’t be afraid to play the moral dilemma card. What if the players discover killing the druid, will unleash a plague. Or discover the druid sent an army of undead dire bears to destroy the town by the river. Now the party needs to decide, what is more important? The druid or the town? Maybe the players will break the first rule of adventuring . Never split the party!
Have fun!