If you control any other creatures as Lord of the Pit’s triggered ability resolves, you must sacrifice one. You can’t choose to be dealt 7 damage instead.

If Lord of the Pit has gained lifelink, being dealt 7 damage by it doesn’t cause your life total to change, even though you gain and lose life. If your life total is less than 7, you won’t lose the game.

If you have two Lords of the Pit, you can sacrifice them to each other.