Atakhan

What happens if I play Atakhan from my deck with Rek'Sai, Breacher on the board? Can I pay the Accelerate cost and kill Rek'Sai for the Atakhan additional cost? Will it enter ready?

Yes, as long as you pay the Accelerate cost, Atakhan will enter ready, even if he kills Rek'Sai for his additional cost. This is because Accelerate creates a delayed replacement effect that causes Atakhan to enter ready. Although he lacks the Accelerate keyword when he resolves, he still has the delayed replacement effect that Accelerate generates.

206. Effects that need to determine a card or token's cost for any purpose always use its printed-or if a copy effect is applied, copied-cost, even if that cost is increased, decreased, or ignored as the card is played. Example: Sky Splitter is a spell that costs [8] and says in part "This spell's Energy cost is reduced by the highest Might among units you control." Lux, Illuminated is a unit that says "When you play a spell that costs 5 or more, give me +3 [M] this turn." If a player who controls Lux plays Sky Splitter, Lux's ability will trigger, regardless of how much was actually paid for Sky Splitter. Example: Defy is a spell which reads "Counter a spell that costs no more than [4] and no more than [A]." Rocket Barrage is a spell that costs [4][C] and has a Repeat cost of [4][C]. Rocket Barrage is a legal target for Defy even if Rocket Barrage's Repeat cost is paid, because Defy only checks the printed or copied cost of its target. Example: Atakhan is a unit that reads in part "You may kill a friendly unit as an additional cost to play me. If you do, I cost [1] less for each Energy it costs and [Y] less for each Power it costs." If a player plays Atakhan and chooses to kill a Reflection token that is currently a copy of Noxus Hopeful, Atakhan will cost [4] less.