With probable vulnerability of undead, may be a good idea...
Umbrils echo moves silently to completly box in the undead creature. Warriors scimitars leap in quick rolling pattern and couple of slashes hit the unnatural hide of the creature. Whether they penetrate or not is hard to say without blood or creatures reaction, but at least there are traces of cuts.
OOC: @mips42 : I know I'm jumping in again, but working weeked looms ahead (big deployment to production)
The creature Seems to mostly shrug off the blows From Bimpnott but turns to face Albrecht when his ax bites into the creatures flesh. Its eyes flash a vivid, blood red for an instant in an attempt to Daze the Dwarf and then lashes out with its' claws.
Umbrils swords do connect but make only minor slits in the creatures hide.
Daze: The targeted creature must succeed on a DC 15 Wisdom saving throw or be charmed until the end of the creatures next turn. While the target is charmed in this way, its speed is halved, and it has disadvantage on attack rolls. Claw attack d20+4 +4=22 to hit, 8 damage to the dwarf.
Getting frustrated with their hardy opponent, Bimpnott drops his sword and with both hands reaches for the undead thing. While the blade splashes into the shallow water, the firbolg's large hands seek to get a grip on the slippery creature.
Making a grapple attempt. This is part of the Attack action, and it's a skill contest: Bimpnott's Strength (Athletics) check versus the opponent's Strength (Athletics) or Dexterity (Acrobatics) (their choice).
At the flurry of blows from Umbril and his Echo, the creature emits a horrible, gurgling, mournful howl. Turning its gaze briefly to the skeletons cowering across the patio, it reaches out withered hands and the skeletons burst into bits! A dark wave of energy flows like smoke across the space and settles into the thing. It then turns its attentions back to the foes in front of it.
It turns once again to Albrecht and opens its mouth wide, sucking the very life out of the Dwarf!
Skeletons are no more.
Drain life: 24= + 4 Damage = 14 Necrotic.