The Chosen Ones

robotworld: Your whole theory is just wrong  .
Firstly, it doesn't make sense to me, but that's a personal opinion and it's not the main problem
Secondly...Dragons? Are you crazy? Well, maybe it could work if you don't use the dragons...

Aero: well, i said pillars as a example, you could put some tree like monsters that can't move, like that "tree like " night elf tower sprite, or maybe some kind of "great wisps".... I think many things would work("pillars,obelisks and temple like" things too in my opinion), but portals are in my opinion already used a lot so you should try to avoid them if possible.

My bad. Yes you are right, my theory is indeed very wrong. I misread the post. Thought I saw epic boss

Here's my revised version

- How does the wisp come?
The boss will summon some forest critters or treants which attack you. Killing them will turn them into wisps. The wisps then move slowly towards the tree boss.

- What happens after X wisps is gathered?
The boss will have increased movement speed/attack for every wisp that reaches the boss. The effect stacks and lasts for 2-3 minutes per wisp.

- How to make this boss spell tactical?
Make it such that when two wisps are in close proximity, they come closer together and disappear in an explosion. So the player may wait for a few whelps to spawn before killing them at one go. Give some delay before the wisps explode so the player can set up protective measures.