Mordekaiser

Dominating's discard is the cost of his trigger, paid as it goes on the chain before the opponent can respond. The Legend can pay one of his two Order Power if a unit or gear is already in your trash. If you exhausted the Legend to help pay for him, a discarded unit or gear waits in your trash until the Legend readies. If the opponent takes the target off the board in response, the kill does nothing, but the discarded card is already in your trash. A unit you cannot afford soon is usually a better discard than a spell you still need.

Once each turn, if Twice Slain would die, you may kill a friendly unit to heal, exhaust and recall him instead. Attacking at eight Might, Assault 3 included, into defenders with six and four Might, he can kill the larger one but takes ten damage, so the save recalls him. The 4-Might defender survives, so the save kept your Champion but did not take the battlefield. Banish and return-to-hand effects cause no death, so they get past the save. Kill a unit card rather than a token when you can, since only the card reaches your trash for the Legend, and avoid killing the only unit guarding another battlefield.

Realm of Death has no Action or Reaction, so it is played on your turn outside Showdowns. It gives an enemy unit -2 Might and a friendly unit +2 Might this turn, then each deals its Might in damage to the other. A 3-Might unit becomes five against a 6-Might enemy cut to four, so the enemy dies and yours survives. Twice Slain is seven here because Assault needs him attacking: he kills anything up to nine Might, but a target of nine or more deals enough to kill him unless you save him. Your unit keeps the +2 and the damage it took, so count both before it attacks.