Shadowverse

With the enhancement, it turns everything on both players' boards into sigils. Only the other "grinder" will be grateful.

Not very legal. On the one hand, it can leave the other player with five sigils that are useless to them. In other words, it's like a "Win" button, as they won't have any followers left. But this niche is already occupied by Seraph, and it's implemented not so clumsily, allowing for the possibility of countering it.
In other situations (that is, in most cases), you simply turn your creatures/amulets into sigils for 10 mana. And you still have to activate these sigils. Questionable usefulness, especially at such a late stage of the game. Moreover, you can go from a situation where "there are followers, but no sigils to activate their abilities" to "turned the followers themselves, who need sigils, into sigils."