Runefather's Boast

Kill a target while it is affected by Runefather's Challenge to gain one Runefather's Boast stack per point of that target's Power. Challenge adds 5 Power, so a normal 20-Power Unique enemy becomes 25 Power and grants 25 stacks. Boast has no duration and can reach 10,000 stacks, but every stack is lost when you die or change area.

Eyes of the Runefather generates the Buff

The generation modifier belongs to Eyes of the Runefather: you gain one Boast per Power of targets affected by Runefather's Challenge that you kill. Applying Challenge alone is not enough unless a source also grants the on-kill Boast modifier.

Each stack grants three flat defences

Every Boast grants +1 Armour, +1 Evasion Rating, and +1 Stun Threshold. For example, 1,000 stacks display as +1,000 to each of those three values before any separate modifiers that apply to the resulting character stats.

The target's Power sets the stack count

The killed target contributes one stack per point of Power. Runefather's Challenge raises that target's Power by 5, so a challenged kill grants five more stacks than the same target would grant without Challenge, before another Power-counting modifier.

A challenged Unique grants 25 stacks

A Unique monster normally has 20 Power. While affected by Runefather's Challenge it has 25 Power, so killing it with the Eyes of the Runefather generation modifier active grants 25 Boast stacks.

The maximum is 10,000 stacks

Runefather's Boast is capped at 10,000 stacks. At the cap it grants +10,000 Armour, +10,000 Evasion Rating, and +10,000 Stun Threshold. Further qualifying kills cannot raise the displayed stack count above that cap.

Boast persists until death or area change

Runefather's Boast has no duration. Its stacks persist while you remain alive in the current area, then all stacks are lost when you die or change area. Entering another map or zone therefore restarts the buildup.

The kill must happen while Challenge is active

Eyes checks for a target affected by Runefather's Challenge when you kill it. Because the Challenge tooltip does not publish its duration and community testing observes about 10 seconds, reapply it before the killing blow rather than assuming an earlier application remains active.