Why is that?
Darkness vs infs is just the thing if you're playing as humans or necros.
...and this also applies to subjugation) specifically in the sense that it's better to use it on melee infs, so even here immunity doesn't solve anything.
And imps can steal mana even without pit lords—it just vanishes into nowhere. So if someone cares (and sometimes they do) about their +1 speed compared to devils, you don't even have to look at what grade of pits are in the army.
p.s. an attack of six cells is powerful, but that's a Dragon Utopia. What a shame :-(
