Merchant of Truth

Flying
Whenever a nontoken creature you control dies, investigate.
Clues you control have exalted. (Whenever a creature you control attacks alone, that creature gets +1/+1 until end of turn for each instance of exalted among permanents you control.)

Ballroom

Ballroom enters tapped.
{T}: Add {W} or {B}.
{4}, {T}: Investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")

Havengul Mystery

When this land transforms into Havengul Mystery, return target creature card from your graveyard to the battlefield.
When the creature put onto the battlefield with Havengul Mystery leaves the battlefield, transform Havengul Mystery.
{T}, Pay 1 life: Add {B}.

Havengul Laboratory

{T}: Add {C}.
{4}, {T}: Investigate. (Create a colorless Clue artifact token with "{2}, Sacrifice this artifact: Draw a card.")
At the beginning of your end step, if you sacrificed three or more Clues this turn, transform Havengul Laboratory.

Havengul Laboratory // Havengul Mystery

Wernog, Rider's Chaplain

When Wernog, Rider's Chaplain enters or leaves the battlefield, each opponent may investigate. Each opponent who doesn't loses 1 life. You investigate X times, where X is one plus the number of opponents who investigated this way.
Friends forever (You can have two commanders if both have friends forever.)

Tamiyo's Journal

At the beginning of your upkeep, investigate. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")
{T}, Sacrifice three Clues: Search your library for a card, put that card into your hand, then shuffle.

Teysa, Opulent Oligarch

Deathtouch
At the beginning of your end step, investigate for each opponent who lost life this turn.
Whenever a Clue you control is put into a graveyard from the battlefield, create a 1/1 white and black Spirit creature token with flying. This ability triggers only once each turn.

Wojek Investigator

Flying, vigilance
At the beginning of your upkeep, investigate once for each opponent who has more cards in hand than you. (To investigate, create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")

Fateful Absence

Destroy target creature or planeswalker. Its controller investigates. (Create a Clue token. It's an artifact with "{2}, Sacrifice this artifact: Draw a card.")

Declaration in Stone

Exile target creature and all other creatures its controller controls with the same name as that creature. That player investigates for each nontoken creature exiled this way.

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.")