Blade of the Ruined King

Q. Can I sacrifice the unit I want to equip Blade of the Ruined King to?

No.

355.16 A player may not make choices during this step that will deterministycally result in illegal choices or actions later in this process unless they have no choice.
_Example: A player plays a card which reads "as an additional cost to play this, kill the unit you control with the most Might. Give a friendly unit +[M] equal to the killed unit's Might this turn. predict 2." They cannot choose to target their unit with the highest Might during this step of finalization.