Fiend Hunter

Mana cost: {1}{W}{W}
Type: Creature — Human Cleric
Power / toughness: 1 / 3
Set: Shadows of the Past #8 (uncommon)

When this creature enters, you may exile another target creature.
When this creature leaves the battlefield, return the exiled card to the battlefield under its owner's control.

Fiend Hunter has 14 printings, first in Innistrad (2011). EDHREC ranks it #3,917 among Commander cards. It appears most in Éowyn, Shieldmaiden decks (5,705 on EDHREC). 3 public decks on TCG run it. It has 4 official rulings.

Legality

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

Public decks that run Fiend Hunter

Evasive Maneuvers - Commander 2013, led by Derevi, Empyrial Tactician
Evasive Maneuvers - Commander Anthology, led by Derevi, Empyrial Tactician
Riders of Rohan - Tales of Middle-earth Commander, led by Éowyn, Shieldmaiden

Commanders that run Fiend Hunter most

Deck counts from EDHREC.

Éowyn, Shieldmaiden: 5,705 decks
Elesh Norn, Mother of Machines: 2,166 decks
Ketramose, the New Dawn: 1,831 decks
Alesha, Who Smiles at Death: 1,759 decks
Orah, Skyclave Hierophant: 1,187 decks
Delney, Streetwise Lookout: 1,107 decks
Preston, the Vanisher: 1,085 decks
Dack Fayden, Helping Hand: 996 decks

Often played with Fiend Hunter

Cards that show up in the same decks on EDHREC.

Rulings

2018-12-07: If Fiend Hunter leaves the battlefield before its first ability has resolved, its second ability will trigger and do nothing. Then its first ability will resolve and exile the target creature indefinitely. This is different from abilities on other cards that exile a permanent "until" something happens.
2018-12-07: Once the exiled creature returns, it's considered a new object with no relation to the object that it was. Auras attached to the exiled creature will be put into their owners' graveyards. Equipment attached to the exiled creature will become unattached and remain on the battlefield. Any counters on the exiled creature will cease to exist.
2018-12-07: If a token is exiled this way, it won't return to the battlefield.
2018-12-07: In a multiplayer game, if you lose the game, the creature exiled with Fiend Hunter remains exiled indefinitely. This is also different from abilities on other cards that exile a permanent "until" something happens.