Partly updated for Enshrouded 1.0— 1.0: the unlock cost; the rest is pre-1.0 wiki data

Merciless Attack
Perform a Merciless Attack by pressing [E] to deal massive damage to an enemy you've empowered. Overpower To overpower an enemy, fill their stun bar by attacking while they block or by parrying their attacks.
Where Merciless Attack sits on the 1.0 tree
In the Core area. It touches a starting point, so it costs 2 points to reach.
Builds that use Merciless Attack
1 preset build tagged MERCILESS_ATTACK — defining picks first, then ranked by tier (S → A → B).
Related Survivor skills
Same archetype — likely candidates to slot next to Merciless Attack in a build.
Fisherman's Resolve1pIncreases your Fishing Endurance by 30%. Allows for extended battles with hooked fish.
Lumberjack1pFelling axes do 30% more damage against wooden objects including trees.
Mason1pThe Pickaxe deals 30% more damage against stone objects, including resource veins.
Opportunity1pValues in the 1.0 files: 40% per rank, 3 ranks.
Power Parry1pEnemy Stun Bar gain from parries is increased by 30%.
Quality Gear1pValues in the 1.0 files: 20%.
Saviour1pYou only need 3 instead of 6 seconds to revive an ally.
Well Rested1pThe base duration for the Rested buff is increased by 5 Minutes. Increase the Comfort in your home to further increase the duration.
Sources: game files 1.0 + Keen Games 1.0 patch notes (changelog, combat changes) for the unlock cost; enshrouded.wiki.gg (pre-1.0) for the description and effects.