Acid Blood. When reduced to zero hit points, the grendelspawn explodes in a gurgling mess of gore and acid. Creatures in melee with the grendelspawn must make a DC 16 Dexterity saving throw, take 35 (8d8) acid damage on a failed save, or half as much damage on a successful one.   Corrosive Blood. A creature that pierces the grendelspawn with a weapon made of metal or wood corrodes. After dealing damage, the weapon takes a permanent and cumulative −1 penalty to damage rolls. If its penalty drops to −5, the weapon is destroyed. Ammunition made of metal or wood that pierces the grendelspawn is destroyed after dealing damage. Magical items get a saving throw to resist this effect.  They must roll 10 or more with advantage to avoid taking the penalty.   Keen Sight. Grendelspawn gains advantage to Wisdom (Perception) checks that rely on sight.   Keen Smell. Grendelspawn have advantage on Wisdom (Perception) checks that rely on smell.   Rage. When a grendelspawn takes damage in combat, it flies into a berserk rage at the start of its next turn, clawing and biting madly until either it or its opponent is dead. An enraged grendalspawn gains advantage on attack rolls while attackers gain advantage attacking it. It also fights for one more round after being reduced to 0 hit points.   Spider Climb. The grendelspawn can climb difficult surfaces, including upside down on ceilings, without needing to make an ability check.