Assassins were batched along with mercenaries, pirates, rogues, and warlocks into outlaws as of Outlaws of Thunder Junction , creating a typal grouping reminiscent of the party creatures
Of course, these cards are legal to play in other Eternal formats like Legacy and Vintage as well, which WOTC also considers from a balancing perspective
With this card and Blight Titan showing up recently, I wonder if well also see more additions to this cycle in the other colors outside of blue and black
Occasionally some decks will require more, while some formats (or deck archetypes) feature enough mana acceleration and cheap spells that they require less
When that ability resolves, the Siege's controller exiles it and may cast it transformed without paying its mana cost
And all that for one mana