Dark Impostor

Mana cost: {2}{B}
Type: Creature — Vampire Assassin
Power / toughness: 2 / 2
Set: Avacyn Restored #92 (rare)
Price: $0.39 USD

{4}{B}{B}: Exile target creature and put a +1/+1 counter on this creature.
This creature has all activated abilities of all creature cards exiled with it.

Dark Impostor has 3 printings, first in Avacyn Restored (2012). EDHREC ranks it #11,287 among Commander cards. It appears most in Ramses, Assassin Lord decks (733 on EDHREC). 2 public decks on TCG run it. It has 6 official rulings.

Legality

commander: legal
legacy: legal
vintage: legal
pauper: not legal
modern: legal
pioneer: not legal
standard: not legal
brawl: not legal

Public decks that run Dark Impostor

Vampiric Bloodline - Crimson Vow Commander, led by Strefan, Maurer Progenitor
Vampiric Bloodlust - Commander 2017, led by Edgar Markov

Commanders that run Dark Impostor most

Deck counts from EDHREC.

Often played with Dark Impostor

Cards that show up in the same decks on EDHREC.

Rulings

2012-05-01: If an activated ability of a card in exile references the card it's printed on by name, treat Dark Impostor's version of that ability as though it referenced Dark Impostor by name instead. For example, if Marrow Bats (which says "Pay 4 life: Regenerate Marrow Bats") is exiled with Dark Impostor, Dark Impostor has the ability "Pay 4 life: Regenerate Dark Impostor."
2012-05-01: Once Dark Impostor leaves the battlefield, it will no longer have the activated abilities of the creature cards exiled with it. If it returns to the battlefield, it will be a new Dark Impostor with no connection to those exiled cards.
2012-05-01: Dark Impostor gains only activated abilities. It doesn't gain triggered abilities or static abilities. Activated abilities contain a colon. They're generally written "[Cost]: [Effect]." Some keywords are activated abilities; they have colons in their reminder text.
2012-05-01: Dark Impostor can target and exile a permanent that's only a creature temporarily, like an animated land. However, because that card isn't a creature card, Dark Impostor won't have any of that card's activated abilities.
2012-05-01: If you control more than one Dark Impostor, each will have only the activated abilities of creature cards exiled with that specific Dark Impostor.
2021-09-24: If Dark Imposter exiles a double-faced card, it will only have activated abilities of the front face of that exiled card, no matter which face was up on the battlefield.