Minecraft Junkie

If i was to spawn a wither and it would escape to the over world Would it be able to destroy other people's houses or is it unable to destroy blocks

I think it can withers can break blocks

Withers can and will definitely break houses.

so if one was to escape would i get blamed for griefing

Most likely yes

If you feel your either is starting to get away from you, please yell into chat, I'm sure people will come to help.

I'm pretty sure they'll despawn if you log out like regular mobs do in SMP. And if not, then LogBlock will likely have a rollback option like with the Ender Dragon. And if not, as long as you're rather far from others' structures, or underground deep, it's not going to do much damage at all. It's really not that big a concern.

To answer the original question, this server does not actually have any way for mobs to travel from the nether into the overworld - to get to the nether is a regular portal, but notice the way back is different. Even if a mob did somehow get through it, the spawn island is immutable and would probably despawn the wither on its own account. I have seen a wither in the overworld before, and it did NOT despawn, in fact it's very likely still out there somewhere.

But unless you put the right thing to its root.
It never will show much flower or fruit.
But I may be one who does not care
Ever to have tree bloom or bear