I believe with Dom 2 it is a -2 defense for each attack (according to the manual). This makes swarming very effective vs high defense units.
The spell that summons 5 magma children for 2 fire gems is pretty good with Argatha. 1 out of 4 earth readers can cast the spell... and of course you can get the flame oracle if you want a guarantee.
I'm curious if it would even be worth taking a heat scale to make better use of these guys. Its a real shame they are not sacred or could get really interesting
You can hit Conj3 on the 7th-9th turn if you get an awake pretender. Or much faster if you build a sage for this purpose (then the sage can site search and help bring in all the gems needed to mass produce the units).