In my recent game I cast the Shadow World spell. As described, monsters come out. It can be (mis)used as an unlimited source of XP, iff you have some levels under your belt. With correct placement of the gate, it can also severely cramp your enemy's style as every army nearby is attacked.

My technical gripe with this spell is twofold; 1) You can close the shadow gate by walking into it, however the you still have to pay for the upkeep (5 mana) for the rest of the game. Closing the gate should turn the upkeep off. I found no way of turning of the upkeep. 2) Casting two of the spells does not seem to double the monsters, i.e. the frequency of monster appearance is constant, no matter how many gates there are. I might be mistaking in this, but it seemed that way.

/Bark