Vargard Obyron's Ghostwalk Mantle ability from the 8th edition Necrons codex which allowed him to teleport every turn to anywhere more than 1" away from enemy units and within 6" of another Necron named character Nemesor Zahndrekh and he could bring a Sautekh Infantry unit with him every time he did it.
You could use the Veil of Darkness relic with an Overlord to bring Zahndrekh close to the enemy and then bring Obyron and another unit within 4" of the enemy, guaranteeing a charge. It didn't even count as falling back, so it allowed a unit to fall back and shoot every turn.
Orikan the Diviner's Temporal Snares from the 5th edition Necrons codex, it made the entire table difficult terrain for the opponent on the first turn, it could be combined with the Writhing Worldscape C'tan power to make it very dangerous terrain. Just a cute little combo for people to get excited for and try to use.
The truth is that I loved 8th edition Quantum Shielding for Necrons, I wanted it gone, it either too good or not good enough, but god damn was I good at making it too good with my rolls.
Detachments costing CP in 9th.
Deep Strike, Pile In, Consolidation and Fall Back rules in 8th. I don't want my units to work on AI or my important unit to randomly disappear, I don't particularly wish the disappearance of a unit of Terminators on my opponent either.