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Varus + Ethereal Weapon
Varus W stacks up to 3 layers of passive on enemies for max-HP magic damage — at ~600 AP, 3 stacks = 100% HP magic damage. Need magic pen. Max W, level E and Q evenly with priority on E. Forge: magic pen > AD (w/ Adapt) > AP > MS. Ethereal Weapon + Nashor's lets ability damage proc passive twice. Weave autos: aq, aw, ar, wq. W detonation damage scales with Q charge time (up to +50%). W active adds bonus damage — max damage = R, W, max-charge Q to detonate 3 stacks.
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Kayle S Auto attack flame waves trigger Ethereal Weapon. 
Aurora S Q+E landing directly stacks 6 layers, triggering the passive twice. 
Kai'Sa S Ethereal Weapon + Duskblade. After upgrading W, one hit procs all plasma stacks. 
Ekko S Ethereal Weapon + Duskblade. Single ability triggers third-hit passive and W passive. 
Rumble S In Danger Zone, Q and E proc the passive damage. 
Lux S Abilities directly trigger the passive. 
Ezreal S Q triggers on-hit effects twice. 
Naafiri S Q bleed and dogs each proc Ethereal Weapon. Pair with Blade of the Ruined King. Keep exploring
This S+-tier combo pairs Varus with Ethereal Weapon in ARAM: Mayhem. Generally not recommended — there are better options.
Varus W stacks up to 3 layers of passive on enemies for max-HP magic damage — at ~600 AP, 3 stacks = 100% HP magic damage. Need magic pen. Max W, level E and Q evenly with priority on E. Forge: magic pen > AD (w/ Adapt) > AP > MS. Ethereal Weapon + Nashor's lets ability damage proc passive twice. Weave autos: aq, aw, ar, wq. W detonation damage scales with Q charge time (up to +50%). W active adds bonus damage — max damage = R, W, max-charge Q to detonate 3 stacks.