Two mana, one black, search your library for any card, put it into your hand — printed in Alpha in 1993, and still the gold standard every other tutor gets measured against.
Combo decks. Finding the missing half of a two-card combo on demand is close to Demonic Tutor's ideal job — turning a dead draw step into a guaranteed answer.
Control and reactive decks. Finding the specific answer a game state calls for — a board wipe, a piece of removal, a counterspell — is exactly what a two-mana any-card tutor is for.
Any deck with a single best card. A deck built around one specific bomb (a wincon, a lock piece) gets far more consistent the moment it can fetch that card on demand.
How it does in Commander
As the commander
It's a sorcery, never a commander — its whole function is sitting in the 99 of a black deck as pure consistency.
In the 99
About as close to an automatic include as black gets: cheap, unconditional, and useful in literally any deck shape, from aggro to control to dedicated combo.
Formats it's strong in
cEDH and combo-focused decks. Consistency is everything at that speed, and finding the missing combo piece on curve is often the difference between winning turn three or not winning at all.
Any black deck. There's no archetype in black this doesn't improve — it's a universal consistency upgrade rather than a strategy-specific card.
Formats it's weak in
Decks with no single best target. A tutor is only as good as what it finds — a deck with no bomb worth fetching gets less mileage than one built around a specific plan.
Very low-power or precon-level pods. Unconditional tutoring can accelerate a deck well past the table's expectations, which is again a Rule 0 conversation worth having.
Synergistic cards
Vampiric Tutor — The instant-speed sibling, for finding an answer at the moment it's needed rather than on your own turn.
Thassa's Oracle — A classic Demonic Tutor target the moment a library-emptying combo is one card away from completion.
Sheoldred, the Apocalypse — One of the most commonly fetched black bombs — a single card that swings a game on its own once found.
Any combo piece — Demonic Tutor's real synergy is with whatever your deck's single most important card is, whatever that happens to be.
Is it worth the price?
Demonic Tutor's age and limited reprint history have kept it a genuinely expensive staple. It's worth the investment in a deck that leans on consistency to function — a combo or control shell — and a luxury upgrade in a more casual, less tuned list.