Like the Winchester brothers, Supernatural's Castiel is also no stranger to death. Ever since his introduction in Season 4, the angel has died at least five different times. He has been blown to smithereens by Lucifer and, most recently, died at the hands of the reaper April Kelly. So it's safe to say that Misha Collins’ Castiel has made his fair share of sacrifices since his debut (a far cry from his initial loyalty as a soldier of Heaven).
To add to the ever-growing number of potential deaths, Castiel ordered a Winchester special and made a deal with The Empty last season and co-showrunner Andrew Dabb promises that Castiel will have to deal with that in Season 15, saying:
I think given what’s happened at the end of last season to Jack… the last thing Cas is worried about right now is [making good on that deal]. But it is something that’s going to come back for us this season. As we saw at the end of last season, The Empty is there. The Empty is one of our probably four cosmic players in this universe. He’s going to have a say.
For background info: Season 14 saw Jack die and go to Heaven, but Castiel (lovingly nicknamed Cas) was convinced that the nephilim couldn’t stay there. Without Jack back on Earth, chaos would reign and thousands (maybe millions) of people would die. However, there was a consequence to bringing Jack back: Cas made a deal with the Shadow to save him from being dragged back to The Empty forever.
Cas convinced the Shadow that it could take him instead of Jack (even though Cas was going to wind up there anyway). A soul for a soul. It's definitely a callback to what Dean did for Sam all those years ago. So long as Cas stays alive, though, the less he’ll have to worry about it. Still, the deal is obviously going to come back and haunt him in a big way, per what Andrew Dabb told to TVLine.
It seems Cas can’t catch a break. Given that Supernatural is entering its 15th and final season, the stakes are much higher than usual for all involved. Previously, Andrew Dabb teased that Dean and Sam were going to lose people “in a real way,” implying that future character deaths will likely be final. That’s a major blow for a show where resurrections have become par for the course. Die, come back to life, rinse, repeat.
Even if Cas made a deal with The Empty, there are probably ways to get out of it. After all, Dean was sentenced to eternity in Hell, but he was saved from eternal damnation by Cas. Though Team Free Will has bigger things to worry about heading into Season 15, Dean and Sam will no doubt work to get Cas out of his deal, consequences be damned.
Of course, the trio will have to deal with Cas lying to Sam and Dean about Jack’s problems first (he killed Mary, oops) before they can figure out a plan to save him from his deal with The Empty. Still, it's nice to believe that everything will work itself out in the end.