Does activating Nasus' Q count as 'using an ability'? Or does it not count until after the Q is actually applied. What i'm wondering is is the gauntlets passive and the effect of the Q are applied to the same attack.. easily tested I know but I figured this would be quicker
It does, and spellblade is applied on the autoattack that applies siphoning strike.
Nasus Q works with all Sheen effects (Sheen 
, Trinity force 
, Iceborn gauntlet 
, Lich bane 
)
And all sheen effects activate the moment you activate an ability (not when it finishes) so the next auto attack will deal increased damage or things like 
Parrrley or other abilities that apply on-hit effects will deal bonus damage from sheen
(oh and sheen is not triggered by toggles and such)
thank you :)
