Rumble + Ethereal Weapon

Vulnerability + Shadowflame creates an 8x multiplicative damage amp with Prismatic.

Void Rift deals magic damage, so it fits AP champs with high magic pen — % magic pen amplifies it, and Pridestalker's ability damage procs it many times. Forge: magic pen > AP > MS.

Rumble's Q and R are extremely high-frequency DoTs. With Jeweled Gauntlet + Vulnerable + Infinity Edge + Shadowflame, each burn tick gets amplified 7.17x — annihilates even tanks. As an AP champion, Jeweled Gauntlet's AP-to-crit ratio is excellent — late, you hit full crit without crit items.