After Ancient Hellkite is declared as an attacker, or is put onto the battlefield attacking, its second ability may be activated. Unless Ancient Hellkite leaves combat, it continues to be attacking (so its ability may still be activated) through the end of combat step.
Random a card of Magic: The Gathering (MTG), but you can also roll the D20 dice, random a Planeswalker, have fun with the random rules and more.