How do i restrict ritual to terrain?

[Sombre]

Hmm. Forest only,....

Can't think of one off the top of my head. That restriction was only introduced relatively recently though I believe, so you'd only have to look through spells recently added to vanilla.

If you're talking about a summoning spell, you could give the monster it summons a forestshape which is the thing you want summoned, and a #firstshape which is something crappy, like a militia. That way if you summon it outside a forest it's just a militia, but if you summon it in a forest it can be something powerful.