Effect triggers after attacking during the Battle Phase
While arguably not the game's primary antagonist - that would be the maester Seymour in my opinion - Sin is a destructive force that feels almost impossible to contend with and provides an incredible story device for the characters
When this happens, the next turn player automatically becomes the commander (unless a different player killed them with combat damage, in which case the monarch moves the same way it normally would)
On top of that, two wildcard slots (one foil, one non-foil) offer more chances to pull cards of any rarity, including high-value mythics.
This hasnt happened (since the cost of a Standard deck has decreased)
According to TCGPlayer , the special Obsidian Flames cards price is set around $49.97, making it significantly more valuable than all the other cards in the set so far