Whenever one or more cards leave your graveyard, you may create a 1/1 black and green Insect creature token, then put a number of +1/+1 counters on it equal to the greatest mana value among those cards
Roon of the Hidden Realm blink your own creatures for extra enter-the-battlefield value, and use his ability with instant-speed interaction (like Tales End or Repudiate) to permanently exile your opponents commanders by stopping them from returning to the battlefield, completely removing them from the game
For these decks, the opportunity of playing Jegantha, the Wellspring is just too high, not because of the number of cards they can't play, but because of the power-level of the small number of cards that don't meet Jegantha, the Wellspring companion restriction
While pitching this and immediately returning it to play with Reanimate or Not Dead After All is backbreaking, Grief is also just a strong card on its own