This is the RDU Token Druid with Onyxia list he was playing on stream. It’s a variation on the extremely popular Token Druid lists that are all over ladder right now.
Onyxia is a powerful finisher and a card that synergizes with several other spells in the deck. It’s being included instead of Yogg-Saron as it’s a lot more reliable.
If you don’t have Onyxia just put Yogg-Saron in. If you really don’t like the card you can run Ragnaros instead as a strong late-game card.