The target of the ability is chosen when Ashling's ability is put on the stack. When the ability resolves, the player dealt combat damage by Ashling sacrifices that creature if they still control it. Neither regeneration nor indestructible can save a creature from being sacrificed.
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.