Where does Rev-Up come from in TFT Set 17?

Rev-Up is a Factory New crate item.

Rev-Up

Gain 8% Attack Speed for each attack against the same target, up to 80%.

graves

Gain 8% Attack Speed for each attack against the same target, up to 80%.

Attack Speed

Bonus Attack Speed makes a unit attack more often per second, which means more auto-attack damage and faster mana generation toward the next ability cast. Some carries scale beyond that — their ability itself fires more hits when they have more Attack Speed (e.g., Bel'Veth's slash count, Jinx's rocket count). Stacking AS items and traits on these units compounds: faster autos, faster casts, and bigger casts.
On Hit

Effect fires on each basic-attack hit — every auto procs items like Red Buff (burn/wound), Nashor's Tooth (mana on attack), and Kraken's Fury (stacking Attack Damage per attack, up to 15). On-hit carries scale with attack-speed buffs, so stacking AS multiplies their damage and utility per second.