Graham O'Brien

A triggered ability that triggers when a spell is cast from anywhere other than your hand, such as that of The Thirteenth Doctor, functions only on the battlefield, so it doesn't trigger when you cast that spell from a zone other than your hand.
If your Commander deck has two commanders, you can include only cards whose own color identities are also found in your commanders' combined color identities.
Paradox abilities count any spells cast from zones other than your hand. These are usually spells cast from exile, the graveyard, or the command zone. They also count spells cast from outside the game, such as spells cast with Wish or Garth One-Eye's ability.
An instant or sorcery that counts how many spells you've cast from anywhere other than your hand, such as Surge of Brilliance, counts itself if it was cast from a zone other than your hand.
If something refers to your commander while you have two commanders, it refers to one of them of your choice. If you are instructed to perform an action on your commander (e.g. put it from the command zone into your hand due to Command Beacon), you choose one of your commanders at the time the effect happens.
If a spell or ability allows you to copy a spell on the stack but doesn't specify that the spell is cast, that spell wasn't cast and won't be counted by paradox abilities. However, if a spell or ability allows you to cast a copy of a spell, that spell will be counted for paradox abilities.
Although Doctor's companion is a new variant of the partner ability, the rules for partner have not otherwise changed. Notably, Time Lord Doctors and cards with Doctor's companion do not interact with cards which have another partner ability.
An effect that checks whether you control your commander is satisfied if you control one or both of your two commanders.
Both commanders start in the command zone, and the remaining 98 cards (or 58 cards in a Commander Draft game) of your deck are shuffled to become your library.
The Doctor's companion ability allows you to have two commanders if one has the ability and the other is a legendary creature that is a Time Lord Doctor and has no other creature types. Creatures with the changeling ability, for example, can't be a second commander this way.
Once the game begins, your two commanders are tracked separately. If you cast one, you won't have to pay an additional the first time you cast the other. A player loses the game after having been dealt 21 combat damage from any one of them, not from both of them combined.