Darksteel Monolith
Artifact
Indestructible
Once each turn, you may pay 
rather than pay the mana cost for a colorless spell you cast from your hand.
Sergey Glushakov
Commander Masters (CMM)
• #778Rulings
If you cast a card for an alternative cost of {0}, you can't pay any other alternative costs. You can, however, pay additional costs like kicker. If the card has any mandatory additional costs, you must pay those.
(wotc • 2023-07-28)
You must still follow any relevant timing rules for the colorless spell you cast from your hand.
(wotc • 2023-07-28)
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Rulings
If you cast a card for an alternative cost of {0}, you can't pay any other alternative costs. You can, however, pay additional costs like kicker. If the card has any mandatory additional costs, you must pay those.
(wotc • 2023-07-28)
You must still follow any relevant timing rules for the colorless spell you cast from your hand.
(wotc • 2023-07-28)