Their only attack is Clapperclaw, which does middling damage and 10 stress damage. They have low HP, high speed and high dodge, so initially hitting them is a bit of a trail. The Shambler Spawn are the actual attacks in this fight. However, Advance and Withdrawal are also the big gimmick of the shambler fight because both also summon two of these guys: His other two attacks are variants with DoTs: Obdurous Advancement moves the Shambler forward 1 space, deals low damage and causes a bleed proc meanwhile Undulating Withdrawal moves the Shambler back a space, does low damage and does a blight proc. This move can cause a lot of issues in most parties because characters like the Hellion and Leper love to be in front and have no real way to get there quickly. Probably his most dangerous attack is Stentorious Lament, which does 10 stress damage augmented by light level and shuffles your party. The shambler's attacks alone aren't all that threatening. Under either circumstance, this will possibly be the hardest boss bar-none in this game for reasons that will be detailed later. Another way you can fight the boss is at random whenever you run around with your torchlight at pitch black.

The most common one will be finding a curio called The Shambler's Altar, which does nothing on its own but summon's this boss instantly if you jam a torch in it. The Shambler is a midboss that you can fight in any dungeon, provided you meet the appropriate circumstances. Part 18: Miniboss Guide: The Shambler The Shambler

Shambler darkest dungeon wiki