The Archmagus was a Starter Universal Prime Helix Card.
How to obtain
The Archmagus was available for all players. This card couldn't be crafted or dismantled.
When creating a new deck, you are given a choice between 3 Prime Helix Cards, this card being one of them. Selecting it will add the card to your deck. You can replace your Prime card at any time by removing it from your deck. The Archmagus does not take a card slot.
The buff triggers only after defeating the Prime Helix Guardian. All allies are affected by their selected Prime Card for 150 seconds.