They have seen sleet storm and wall of ice, that's all. But he's going to use the Ring VERY sparingly, and if I want him to get possessed then I will just have it happen at a dramatically appropriate time, I'm probably not going to roll for it. Dragonbait knows the danger of the Ring, but because he can't talk, that's going to be difficult to explain to the party when Artus wigs out. This is great. Between the Frost Giants after the Ring, and the Red Wizards of Thay (maybe that lich in the Heart of Ubtao knows about him) and Artus succumbing to possession by an utterly inhuman intelligence, I think this will all work out just fine.If Artus leaves a lot of those perma-frozen zones in the tropical jungle, somebody will figure out 2 + 2 = 4 and then the hunt will be on. Artus is tied to a fixed place (Mezro); he can't wander the world at will. Therefore he has to be stealthy in a strategic sense: not leave a lot of signs of his presence, let alone his position. He will be most reluctant to use the powers of his Ring in front of witnesses, and he will only use the things that never melt because he or Dragonbait are going to die if he doesn't.
If you haven't already, don't use most of the Ring's powers. Maybe the PCs will find that Random Encounter with a frozen zone after they meet Artus (who can predict the dice?), and figure it out themselves. But he should be played as a "sorcerer" (of approximately the party's level) dependent on his focus instead of his blood. He should not behave like 'Arctus Cimber, King of Winter' in a decidedly not-winter setting.