Darkland Treant

[Mesh Hong]

The players, in the encounter they fought, knew were the area was and simply walked around it, so it felt like it kind of defeated the purpose. Thereby me thinking that if the area was bigger, they might be forced to walk into it a few times, taking the damage. I'd take out the ongoing if making it bigger doesn't break the balance, what do you think?

The tactic I would usually try to use with this sort of effect is to have the creature move into the zone it has created, it usually should be immune to the damage. This forces the PCs to go into the zone to engage the creature in melee, or to use powers that push, pull or slide it out. Either way they have to deal with it and not just ignore it.