It grants the use of a stealth move that, when used next to, above, or below the target, will send Talion into a fit of rage, where he stabs, slices, and mauls his target, effectively terrifying most Uruk soldiers (2 at minimum) that happen to witness the deed, into running away out of fear.
The fear inducement effect is generally ineffective to Captains & Warchiefs, unless they happen to have a fear of Talion i.e. "Fear of the Gravewalker.".
Brutalize consistently causes exactly 5 hits, which are added to the Hit Streak. If the Blade Master ability has also been obtained, this is sufficient to charge the Hit Streak and allow Talion to immediately transition into another powerful move.