The deck features 3 copies of Angelic accord, which create a strong combo with invasion of dominaria and other spells with life gain effects such as Alseid, Daxos, Inspiring Overseer, Lyra, etc
One of the reasons this card was banned was that the synergies with Manifold Mouse were too strong: They allow you to target your Heartfire Hero for free to trigger valiant and give it a +1/+1 counter, while you also give the Hero double strike for a turn
Its size ensures that the card is perfectly protected in the event of external damage
In the case of our Death's Shadow list, the deck is playing 43 nonlands, so we can divide 33 (the total CMC) by 43 (the number of nonlands) to find the average converted mana cost of the nonland cards in the deck
Additionally, it also comes with the added benefit of allowing its controller to pay three mana in order to provide all creatures they control with +0/+1