A 3-mana colorless rock that taps for three and untaps itself for three — which nets zero on its own, and nets infinite the moment anything makes untapping it cheaper or free.
Rings of Brighthearth mana doubling. Copying the untap ability for {2} turns Basalt Monolith into a repeatable, if slow, mana-positive engine.
Power Artifact infinite mana. Reducing the untap cost to zero makes the rock generate unlimited colorless mana on its own — a dedicated two-card combo.
Straightforward ramp. Tap for three, untap for three later in the same turn: a real way to jump from three mana to a six-mana play in one turn without any combo pieces at all.
How it does in Commander
As the commander
Not a commander — it's an artifact with no legendary creature version, so it only ever shows up as a mana rock or combo piece in someone else's deck.
In the 99
A staple-tier mana rock in colorless-friendly decks and a genuine combo enabler in artifact or high-power shells running Rings of Brighthearth or Power Artifact, without being a dead card in a list that never draws either.
Formats it's strong in
Artifact-heavy and colorless ramp decks. It does its job as a rock even before any combo piece shows up, so it's never a wasted slot.
cEDH pods running Power Artifact. Those decks are specifically built to find and protect this two-card infinite mana line.
Formats it's weak in
Decks with no way to use a mana burst. Tapping for three all at once is wasted if there's nothing worth spending it on that turn.
Low-power tables with no combo pieces around it. Without Rings of Brighthearth or Power Artifact, it's simply a solid rock — good, but not the reason to build a deck.
Synergistic cards
Rings of Brighthearth — Copies the untap ability for {2}, netting extra colorless mana every activation.
Power Artifact — Reduces the untap cost to {0}, making Basalt Monolith an infinite colorless mana source by itself.
Sol Ring — Another cheap rock that helps get to the mana needed to activate Basalt Monolith more than once a turn.
Metalworker — A creature that turns extra untapped artifacts, Basalt Monolith included, into an explosive mana burst of its own.
Is it worth the price?
Basalt Monolith is inexpensive and worth owning as a mana rock alone, before any combo consideration. Power Artifact carries most of that line's price and rarity, so the combo itself is the more expensive half of the equation.