Curious Inquiry

Enchant creature
Enchanted creature gets +1/+1 and has "Whenever this creature deals combat damage to a player, investigate." (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")

Ethereal Investigator

Flying
When Ethereal Investigator enters, investigate X times, where X is the number of opponents you have. (To investigate, create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")
Whenever you draw your second card each turn, create a 1/1 white Spirit creature token with flying.

Unshakable Tail

When Unshakable Tail enters and at the beginning of your upkeep, surveil 1.
Whenever one or more creature cards are put into your graveyard from your library, investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")
{2}, Sacrifice a Clue: Return Unshakable Tail from your graveyard to your hand.

Offer Immortality

Target creature gains deathtouch and indestructible until end of turn. (Damage and effects that say "destroy" don't destroy it.)

Bearer of Overwhelming Truths

Prowess (Whenever you cast a noncreature spell, this creature gets +1/+1 until end of turn.)
Whenever Bearer of Overwhelming Truths deals combat damage to a player, investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")

Daring Sleuth

When you sacrifice a Clue, transform Daring Sleuth.

Daring Sleuth // Bearer of Overwhelming Truths

Homicide Investigator

Whenever one or more nontoken creatures you control die, investigate. This ability triggers only once each turn. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")

Drag the Canal

Create a 2/2 white and blue Detective creature token. If a creature died this turn, you gain 2 life, surveil 2, then investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")

Floodhound

{3}, {T}: Investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")

Persuasive Interrogators

When Persuasive Interrogators enters, investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")
Whenever you sacrifice a Clue, target opponent gets two poison counters. (A player with ten or more poison counters loses the game.)

Murder

Forensic Gadgeteer

Whenever you cast an artifact spell, investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")
Activated abilities of artifacts you control cost {1} less to activate. This effect can't reduce the mana in that cost to less than one mana.

Erdwal Illuminator

Flying
Whenever you investigate for the first time each turn, investigate an additional time.

Mistway Spy

Flying
Disguise {1}{U} (You may cast this card face down for {3} as a 2/2 creature with ward {2}. Turn it face up any time for its disguise cost.)
When Mistway Spy is turned face up, until end of turn, whenever a creature you control deals combat damage to a player, investigate.

Gone Missing

Put target permanent on top of its owner's library.
Investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")

Battle-Rage Blessing

Lazav, Wearer of Faces

Whenever Lazav, Wearer of Faces attacks, exile target card from a graveyard, then investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")
Whenever you sacrifice a Clue, you may have Lazav become a copy of a creature card exiled with it until end of turn.

Foul Play

Destroy target creature with power 2 or less. Investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")

Eater of Virtue

Whenever equipped creature dies, exile it.
Equipped creature gets +2/+0.
As long as a card exiled with Eater of Virtue has flying, equipped creature has flying. The same is true for first strike, double strike, deathtouch, haste, hexproof, indestructible, lifelink, menace, protection, reach, trample, and vigilance.
Equip {1}

Hard Evidence

Create a 0/3 blue Crab creature token.
Investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")

Hold for Questioning

Enchant creature or planeswalker
When Hold for Questioning enters, tap enchanted permanent and investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")
Enchanted permanent doesn't untap during its controller's untap step and its activated abilities can't be activated.

Trail of Evidence

Whenever you cast an instant or sorcery spell, investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")

Hostile Investigator

When Hostile Investigator enters, target opponent discards a card.
Whenever one or more players discard one or more cards, investigate. This ability triggers only once each turn. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")

Bilious Skulldweller

Deathtouch
Toxic 1 (Players dealt combat damage by this creature also get a poison counter.)

Argivian Avenger

{1}: Until end of turn, Argivian Avenger gets -1/-1 and gains your choice of flying, vigilance, deathtouch, or haste.

Agent of the Shadow Thieves

Commander creatures you own have "Whenever this creature attacks a player, if no opponent has more life than that player, put a +1/+1 counter on this creature. It gains deathtouch and indestructible until end of turn."