Here is the full list of all 10 Runes of Brennos achievements.
Finish your druid training.
Perform the Energy rune combo.
Perform the Elemental Rage combo.
Perform the Elemental Shield combo.
Discover and perform the Absorb combo.
Unify Gaul under your dominion.
Fight 100 rune battles.
Win 30 rune battles.
Win 100 rune battles.
Play for five hours.