The corridor is forty feet long and eight feet wide. On the left, iron bars run floor to ceiling with a troll behind them. On the right, same setup, second troll. The gap between the bars on either side is eight feet, but when the trolls reach through the bars and into the corridor, there is only a one foot wide gap between their clawed hands. One person, moving quickly, has a chance to thread through without getting grabbed. Two people moving at the same time cannot. The trolls aren’t coordinating their actions, they’re just enraged and both trying furiously to grab the adventurers.
Any character who moves through at their highest possible speed must win a contested check, their Dexterity (Acrobatics) against the troll’s Strength (Athletics) +7, or get grabbed and pulled against the bars. A grabbed character takes 2d6+4 slashing damage at the start of each of their turns until they break free (another contested check on their turn). A character who moves slowly must roll their Dexterity (Acrobatics) check with disadvantage.
After at least one character makes it to the far side, one of the trolls tears off its own arm at the shoulder and throws it into the middle of the corridor. The arm acts on initiative count 20, moves 20 feet per round, and makes a grapple attempt (+4 Strength Athletics) against any creature within 5 feet. A grabbed creature is restrained until they win a contested Strength (Athletics) check on their turn. The arm has AC 12 and 10 hit points, and regenerates 5 hit points at the start of each round unless it has taken fire or acid damage. The “donor” troll regrows the arm in 1d4 rounds.
If at least two party members have made it through the corridor, the other troll rips its arm off and throws it into the corridor as well.
The severed limb(s) change everything. Fire is the obvious answer, and fire in a narrow corridor between two trolls is its own problem. Acid works just as well. If the arm takes fire or acid damage it stops regenerating, and once it’s destroyed it stays down. If the party ignores it, the arm keeps grabbing, the troll’s arm grows back, and the troll throws that one too.
Note: This is a key encounter inside a dungeon I ran for a long-time group of 5e players. In my encounter, I had four trolls, each in their own cell, two on each side of the corridor. The party used fire (which stank to the Seven Heavens) to destroy the trolls before any attempted to pass through.
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