Magnar race. Slaves. Flesh Bound Tome. Cull the Weak.
Train a bunch of super cheap slaves, stack them with a Sov/Champ, attack the weakest lair you can find, then Cull, Cull, Cull. Kill the monster(s), pocket the extra mana, cycle, rinse, repeat at another lair. Getting lots of mana very early in the game is extremely beneficial.
Is there a better way of generating mana other than eating the Shards via the Consume spell?