Sanctum of Eternity

• WWizards of the Coast2019-08-23
  Official Ruling
  The “commander tax” increases based on how many times a commander was cast from the command zone. Casting a commander from your hand doesn’t require that additional cost, and it doesn’t increase what the cost will be the next time you cast that commander from the command zone.
• WWizards of the Coast2019-08-23
  Official Ruling
  If your commander isn’t on the battlefield (or if you’re not playing the Commander variant), Sanctum of Eternity’s second ability does nothing.

Rulings via Scryfall