I like the suggestions that other people have, but I'd go with a Shambling Mound (a CR 5 monster).
They're basically giant plant monsters, and normally wouldn't be able to corrupt a forest but it introduces a concept that I like to use.
Make the BBEG a minion. Yes, he's big, bad, and will swallow half your party whole but he's just someone else's lackey. The real threat is hidden off-screen somewhere and you can face other lackeys in future adventures if the group takes the bait.
As far as planning for the ultimate BBEG, you don't need to do much at this point other than drop hints that someone else is behind this nefarious plot to corrupt the land. Killing the shambler is a worthy goal and by doing so you win the battle only to realize that you are now at war.
I'd have the ultimate BBEG deliver the standard bad-guy monologue after the adventurers have won. Something along the lines of "Incompetent fools, this was just one of my weaker subjects. You have merely succeeded in getting my attention, soon you shall pay! BWAHAHAHA!"
If the group doesn't seem to care you can abandon the plot. If they take the bait you have a lead in to future adventures.
They're basically giant plant monsters, and normally wouldn't be able to corrupt a forest but it introduces a concept that I like to use.
Make the BBEG a minion. Yes, he's big, bad, and will swallow half your party whole but he's just someone else's lackey. The real threat is hidden off-screen somewhere and you can face other lackeys in future adventures if the group takes the bait.
As far as planning for the ultimate BBEG, you don't need to do much at this point other than drop hints that someone else is behind this nefarious plot to corrupt the land. Killing the shambler is a worthy goal and by doing so you win the battle only to realize that you are now at war.
I'd have the ultimate BBEG deliver the standard bad-guy monologue after the adventurers have won. Something along the lines of "Incompetent fools, this was just one of my weaker subjects. You have merely succeeded in getting my attention, soon you shall pay! BWAHAHAHA!"
If the group doesn't seem to care you can abandon the plot. If they take the bait you have a lead in to future adventures.