I often point out that Rage itself could easily be refluffed as “zen focus” or “bullet time,” since all it really does it make you tougher and hit more and harder. There is the thing about not being able to use Concentration, Combat Expertise, or anything that requires patience, but for most warriors those things don’t come into play in the first place, so you can just quietly not do those things while roleplaying as if you could, you’re just not because you’re focused on killing your target.
But if you want something that is explicitly, in the books, more related to such “cold” rage, I’m not aware of anything specifically like Rage, as in a state you enter into.
However, there are lots of various moves and feats that capture that concept pretty well, in my opinion. For examples:
Unnerving Calm from Tome of Battle allows you to use Concentration instead of Intimidate for the Duel of Wills mechanic used in that book. The duel of wills is an optional maneuver you make when rolling initiative, and is a pretty neat effect: you can get some minor, but nice, temporary bonuses for winning or for your opponent forfeiting the duel.
Unnerving Calm is a bonus feat for the warblade, also Tome of Battle, who gets the Diamond Mind and Iron Heart disciplines. Diamond Mind uses Concentration checks for all kinds of things: Concentration check to make a target flat-footed, make a Concentration check as a damage roll, make a Concentration check instead of a saving throw. Meanwhile, Iron Heart is all about the brutal, perfect blow, shrugging off all attempts to stop your onslaught.
Meanwhile, the swordsage class, from Tome of Battle again, gets Diamond Mind and Shadow Hand. That is extremely fitting for an assassin: the Shadow Hand discipline offers shadowy teleports, brief spurts of invisibility, the ability to walk on walls, Sneak Attack damage, and all kinds of nasty effects on attacks. The swordsage gets Wis-to-AC while wearing light armor, all kinds of appropriate assassin-y weapons, and a sweet 6+Int skill points with the skills you want.
In Pathfinder, Dreamscarred Press’s Path of War is inspired by Tome of Battle, so that may also be worth looking at.