I’m familiar with the setting, but I’d recommend not giving players the risk of dying to receive new abilities. As accurate as that is to how it works in the setting, remember that there’s a reason the PC in Origins always survives their joining.
Think about what this would be like for your party. Let’s say you have a group of 4, and 3 of them decide they want to join the Grey Wardens. They take the ritual, and only 2 survive. The surviving 2 are now more powerful than the others (maybe only slightly, but I assume that you want it to be noticeable) and feel like they’re in a special club. The one who failed her resist roll is now miserable, she sees the two who succeeded and curses the luck of the dice. Maybe she makes a new character who you now have to fit into the game, or maybe she thanks you for the game and stops coming. Meanwhile, the fourth player who never took the ritual feels like they missed out. They feel like a second-class citizen in the party and wish they had decided to go for it. Little do they know, if they had taken the ritual they would have also failed their resist roll and lost this character that they love playing to a single, non-combat roll that they took because everyone else was doing it. Not quite the dramatic and meaningful death they hoped for.
My advice would be to make it less accurate in order to make it more fun. Maybe they’re guaranteed to survive, but they have to make a Fortitude roll or be afflicted with a few levels of Fatigue. Maybe it’s something narrative that all the players will be offered, where all the consequences are decided by you. Or maybe it’s purely flavour, if a player wants to start the game as a Grey Warden they can build their character in a way that represents that (up to the player what that means, of course) or if they want to become one later in the game then they can put their Feats and Attributes towards it and only use these new abilities once they have undertaken the joining.