Effect
Increased Break damage and faster Riposte opportunities. The seal changes posture and parry flow rather than simply adding weapon damage.
Riposte opportunities are created by breaking enemy posture through parries, attacks, and effective sidearm use. Slayer Seal makes that loop easier by increasing Break damage and exposing the opening sooner. It is therefore a seal-level change to defensive timing, not merely a cosmetic or weapon infusion effect.
Location
One route follows the Marrow Keep beacon along a forest path to a stone chair. Another description puts it on a corrupted clifftop to the right of Merrick's room. The shared hub is confirmed, but the conflicting landmark wording is preserved.
The safe shared instruction is to search the Marrow Keep area around the beacon and Merrick's route.
Trade-off
Achievements are disabled when the seal is equipped. Whether a later patch changes save behavior.
Both are opinions around the same documented consequence.
Related videos
Watch the route and combat use
The first guide starts muted. Scroll on and it stays in the lower-right corner on desktop; close it at any time.