The Tar Golem is the scripted prologue boss of Mortal Shell II. You do not have to win this fight to continue the story, but beating it down to minimum health unlocks a trophy.
Whether you win or lose, the same cutscene plays: your Shell saves you before being destroyed, Harros the Vassal is killed, and you are left without a Shell before reaching Marrow Keep.
Where to find the Tar Golem
Past the Disciple's Grotto Beacon, take the downward sloping path on the right and use the updraft to reach the upper area. Pass the Elite Brigand (Ruk) and continue to the Bone Gate. Entering the gate triggers the boss arena and cutscene.
How to fight the Tar Golem
Its clap attack followed by a jump cannot be parried, so dodge it. Attacks marked with a red indicator should be parried just before impact. Axe swings can be guarded, and guarding builds stagger that eventually lets you land a Riposte for big damage.
At roughly half health its attacks gain Fire damage, so avoid getting hit. Use your sidearm for burst damage and land melee hits to restore Resolve. Keep melee combos short (about two hits) before guarding.
Rewards and retrying
Reducing the Tar Golem to minimum health unlocks the “No, You Still Can't Win” trophy. One source reports killing it grants Vatra's Seal, though this is unconfirmed. Story progression is unchanged either way.
To retry, run off the ledge in the arena to reset at the nearest Beacon. On PC you can also press Alt+F4 as the ending cutscene begins, then continue to reappear outside the Bone Gate.