How does Ignitis's Battle Lust work?

Two kills put you at the cap: 50% extra damage, lasting 20 seconds. Ignitis earns a stack only by defeating something itself, so the bonus belongs to whichever Aniimo lands the final blow. If a teammate takes the kill, Ignitis gets nothing.

Battle Lust is a passive. You never activate it and it costs nothing. What you do control is who finishes enemies. Let Ignitis take the last hits on weaker targets early in a fight, then turn it on the tougher enemy while both stacks are running. A third kill adds no third stack, because two is the limit.

The same trait appears under other names in different parts of the game, Battle Fervor and Battle Forever among them. All of them describe this one passive, so if you see 25% per kill stacking twice, you are looking at Battle Lust.