MarkB
Legend
If it's an ongoing campaign:Question said:See title. How do we know it is a sphere of annihilation or some kind of portal?
Examine your DM carefully. If he appears to be a complete bastard, it's probably a Sphere of Annihilation. If he doesn't, it's probably a portal.
If it's a 1-off, anything-goes game, you're on your own.