Only the attack damage portion of Siphoning Strike is affected by critical hits. [1]
Siphoning Strike resets the auto-attack counter, meaning you can get an additional swing by using Siphoning Strike after an auto-attack.
Spirit Fire reveals fog of war for a short duration on cast, in a smaller area than the spell's AoE.
Bonus attack damage from Fury of the Sands cannot exceed 300.
Fury of the Sands does not directly heal Nasus but does provide him with both a flat amount of extra health and can provide bonus damage that improves the lifesteal from Soul Eater.
Siphoning Strike will miss if Nasus is blinded, but the buff will not be used.