In UMS I'm trying to make a trigger that creates a DWeb, but nothing I've tried has succeeded. I've looked all over for this and can't find an answer. Please help.
For UMS maps, you should visit Staredit.net, this map is dedicated to melee mapping.
Anyways, I don't think it is possible with the create unit action (check on Staredit.net to make sure). There is, however, a AI script at location "Cast DWeb here", which requires a corsair for the specified player, enough energy and the spell researched.
You can put a few computer controlled corsairs in some corner of the map and order them to cast DWebs there, and then you can MOVE those DWebs freely. You can also kill/remove DWebs. Notice that DWebs are always owned by Player12 (Neutral).