How does the wisp come?
Eh? If it's passive then it would be from nothingness, no cast or some kind of ritual, it would just appear. Oh, maybe some kind of special effect, some light(green light...) or some vines?
What happens after X wisps is gathered?
Hum...they would come for Phodom's life? Or maybe they would sacrifice themselves to heal(and maybe buff) the boss? A super-duper-hyper spell?
How to make this boss spell tactical?
Seriously, i can't understand what tactical means...but my suggestion would be that the wisps would have to be destroyed to avoid more problems(like, duh?) so they should spin kind of fast to avoid phodom's attack the most. Oh, and maybe it should have some kind of penalty when you kill a wisp? Some kind of explosion that damages(but not that much) phodom or a debuff...
Seriously i don't know what tactical means, but, oh well, i have made my suggestions.