It's one of 10 exclusive theme booster rares. Exclusive, except one of the card slots in the collector boosters can have a theme booster card (or the BAB promo, or plansewalker deck "exclusives").
Each of the theme boosters has 1 rare or mythic, which can be any rare or mythic from the normal set in that booster's color, or one of the two exclusives for that booster's color. (Each booster only has one color of cards). So if you buy the red theme booster, you most likely have a 1/10 chance of getting a Terror of Mount Velus (assuming the rares are twice as likely as the mythics).
Or you could just buy it as a single, be guaranteed to get a copy, and most likely spend less money. (Preorder price on Terror of Mount Velus is about the cost of 2 theme boosters.)
I need one for my Zirilan of the claw commander deck
Pioneer:UR Pheonix
Modern:U Mono U Tron
EDH
GB Glissa, the traitor: Army of Cans
UW Dragonlord Ojutai: Dragonlord NOjutai
UWGDerevi, Empyrial Tactician "you cannot fight the storm"
R Zirilan of the claw. The solution to every problem is dragons
UB Etrata, the Silencer Cloning assassination
Peasant cube: Cards I own
Each of the theme boosters has 1 rare or mythic, which can be any rare or mythic from the normal set in that booster's color, or one of the two exclusives for that booster's color. (Each booster only has one color of cards). So if you buy the red theme booster, you most likely have a 1/10 chance of getting a Terror of Mount Velus (assuming the rares are twice as likely as the mythics).
Or you could just buy it as a single, be guaranteed to get a copy, and most likely spend less money. (Preorder price on Terror of Mount Velus is about the cost of 2 theme boosters.)
Two Score, Minus Two or: A Stargate Tail
(Image by totallynotabrony)