Verified for patch 16.8.1

Deadlier Caps

Gain a Rabadon's Deathcap. Rabadon's Deathcaps permanently gain ?% Ability Power whenever their holders score a takedown.

Deadlier Caps enables

On Takedown On the Menu

Effect fires when the source secures a kill or assist.

Deadlier Caps scales with

Ability Power On the Menu

Scales with Ability Power. Damage and effect magnitudes that read "(scales with AP)" target this mechanic.