2019-08-23 : 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.

2019-08-23 : 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.