It does spawn if you fight on the sides. There are some terrain issues on the side where the Mage is, though; sometimes the worm burrows underneat a ledge and doesn't come up where it can be attacked, but it can still attack the player.
Also is it just me or is the Gheist who's with Alexander much smaller than the Gheists we saw with Dallis?
Also where is Dallis, why isn't she with Alexander?
