[Gregstrom]

In my experience, the downside on Lightless Lantern is negligible. In a recent game, I had about 40 of them and lost all of 2 mages to Horrors - a price well worth the paying.

Oh, and you get a better death gem/rp ratio with Lanterns than Mentors. Sure, mentors give a better total gem/rp ratio, but you have an awful lot of stuff you want to spend death gems on.

Btw, for pure efficiency in research, revenants are 3 gems cheaper than spectators and you might want to use them when you have them researched.