Rex, Cyber-HoundHow it works & interactions with other cards

A Master of Mimicry

Rex, Cyber-Hound functions in three stages: generating resources, exiling targets, and gaining abilities. When Rex deals combat damage to a player, that player mills two cards and you gain two energy counters. You can then spend two energy counters as a sorcery to exile a creature card from any graveyard with a brain counter. As long as Rex is on the battlefield, it possesses every activated ability of all cards currently in exile with brain counters on them.

Activated vs. Triggered Abilities

It is crucial to distinguish between activated and triggered abilities when using Rex. Rex only gains activated abilities, which are always formatted as '[Cost]: [Effect]'. If a card in exile has a triggered ability (starting with 'When', 'Whenever', or 'At') or a static ability, Rex will not gain those features. If an ability Rex gains mentions the name of the original card, you treat that text as if it refers to Rex, Cyber-Hound instead.

Strategic Timing

Because Rex's exile ability can only be activated as a sorcery, you must plan your turn carefully. You cannot use his energy-spending ability during combat or in response to an opponent's spell. However, once Rex has gained an activated ability, you can use that new ability whenever its timing rules allow—usually any time you have priority, unless the specific ability states otherwise.

Interaction examples

Combat and Energy Collection

Rex, Cyber-HoundEnergy Reserve

Rex, Cyber-Hound attacks and deals combat damage to an opponent. This triggers Rex's first ability, causing the opponent to mill two cards and granting you two energy counters, which you track using your Energy Reserve. You now have the necessary energy to use Rex's second ability during your second main phase.

Limits of Learning

Rex, Cyber-HoundSolemn Simulacrum

If you use Rex, Cyber-Hound to exile Solemn Simulacrum with a brain counter, Rex gains nothing. Solemn Simulacrum only has triggered abilities ('When this creature enters...' and 'When this creature dies...'). Since these are not activated abilities (they lack a colon), Rex does not inherit the land-searching or card-drawing effects.

Doubling the Data

Rex, Cyber-HoundRoaming Throne

With Roaming Throne on the battlefield naming 'Robot' or 'Dog', Rex's combat damage trigger will trigger twice. This means the opponent mills four cards total and you gain four energy counters instead of two. Note that Roaming Throne only doubles triggered abilities, so it will not double the activated abilities Rex copies from exile.