Whenever Rex, Cyber-Hound deals combat damage to a player, they mill two cards and you get ![Energy](/Handlers/Image.ashx?size=small&name=e&type=symbol)
(two energy counters).
Pay ![Energy](/Handlers/Image.ashx?size=small&name=e&type=symbol)
: Choose target creature card in a graveyard. Exile it with a brain counter on it. Activate only as a sorcery.
Rex has all activated abilities of all cards in exile with brain counters on them.