Mordekaiser Ability (LoL): Darkness Rise
Darkness Rise Mordekaiser
Passive

Damaging abilities or basic attacks grant 1 stack per enemy champion or large monster hit, up to a maximum of 3. After 3 basic attacks or abilities against enemy champions and large monsters, Mordekaiser gains Darkness Rise for 4 seconds, causing him to deal 5-15 0.3) (+1-5% of target's maximum health) magic damage (depending on level) per second to all nearby enemies and gain 3% bonus movement speed (levels 1-18). Damage is capped against monsters at 28-164 (depending on level).
Further basic attacks and abilities against enemy champions and epic monsters refresh Darkness Rise's duration.
I can't 100% confirm this, but I believe it said 8.