Darksteel Monolith

• WOTC Official Reminder Textofficial rules
  Indestructible — Effects that say "destroy" don't destroy this permanent. A creature with indestructible can't be destroyed by damage.

• WWizards of the Coast2023-07-28
  Official Ruling
  If you cast a card for an alternative cost of , 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.
• WWizards of the Coast2023-07-28
  Official Ruling
  You must still follow any relevant timing rules for the colorless spell you cast from your hand.

Rulings via Scryfall