Build around Fey Magic + Ornn's first R hit. Fey Magic procs Brittle 4 times on first ult — Brittle's 18% max-HP magic damage stacks four ways for 72% max HP, then magic pen + amp items finish the kill. Forge: ability haste > magic pen. At ~500 haste, Fey Magic is ~15s CD and ult is ~16s — every fight is a nuke. Build straight magic pen + damage amp.
Submitted by ARAMMayhem.com
Ornn S With Fey Magic, R first cast triggers Vulnerability multiple times. Huge damage. 
Twitch S Ult autos also trigger the polymorph. They can't even fight back. 
Anivia S Use Fey Magic's polymorph for CC-burst damage. Fey Magic + Killing Time = absurd. 
Zeri A After R, Q chain lightning also triggers the polymorph. 
K'Sante A After R, basic attacks trigger the polymorph. Keep exploring
This S-tier combo pairs Ornn with Fey Magic in ARAM: Mayhem. This is one of the strongest combos available — pick it whenever you can.
Build around Fey Magic + Ornn's first R hit. Fey Magic procs Brittle 4 times on first ult — Brittle's 18% max-HP magic damage stacks four ways for 72% max HP, then magic pen + amp items finish the kill. Forge: ability haste > magic pen. At ~500 haste, Fey Magic is ~15s CD and ult is ~16s — every fight is a nuke. Build straight magic pen + damage amp.