The party (5 PCs at Level 6 at the time) was forewarned by an NPC that they would need to prepare for climbing. Taking the hint, they stole three sets of climbing gear and two potions of climbing from a couture sporting goods store (as you do). When it came time to do the actual climb, I made sure they had time to distribute the gear and plan their ascent. One player carries an immovable rod, which I allowed them to combine with rope to improvise a fourth set of climbing gear. I then announced a skill challenge and laid out the rules.

The Rules

1. You don’t have enough rope for parallel ascents, so you’re going on a single line.
2. If you’re harnessed (as with climbing gear), you can’t move past the person next to you, either up or down.
3. If anyone rolls a natural 1 on any skill check, they fall.
4. If someone falls, the PCs adjacent to them get 1 chance each to catch them by succeeding on a DEX 15 saving throw, and a STR 13 saving throw to keep from dropping them. Each PC gets one chance as the falling PC enters their space.

(I didn’t tell them the last one until it became relevant…)

The Skill Challenges

Climbing Fools Pass is divided into three skill challenges, one per hour of climbing. To succeed, a majority of the party (3 of 5) must succeed on Strength (Athletics) checks of increasing difficulty. Since most of my PCs have small (or no, or negative) strength bonuses, the DCs are modest: 13 for the first leg, 14 for the second, 15 for the third and any subsequent checks.

If the party succeeds on all three skill challenges, they reach their goal safely.

If the party fails a skill challenge, it triggers a complication.

Regardless of success or failure, the party reaches the top after 7 attempts.

Complications

On any skill challenge failure, the party must re-roll on the same challenge DC. If they succeed, they move on, no penalty. However:

1. If they fail two consecutive attempts, an encounter is triggered.
2. If they fail an attempt on the third leg, a deadly encounter is triggered.
3. For every attempt after the 4th, the party gains one point of exhaustion when they reach the top: 5 attempts yields one point of exhaustion, 6 attempts 2 points, and 7 attempts 3 points.

Encounters

When two consecutive skill challenges are failed, the party will be attacked by 2 harpies closing from a random direction (top bottom or side). Pick one player to make a Wisdom (Perception) DC 16 check. If they fail, the party is surprised for one round.

When a deadly encounter is triggered, a Chimera nesting at the top of their ascent attacks. The topmost PC must succeed on a Wisdom (Perception) DC 16 check or the party is surprised for one round.

During combat, if a PC is holding onto the rope rather than harnessed to it, they cannot use two-handed weapons or dual-wield. Crossbows, firearms, etc. cannot be reloaded.

All PCs who aren’t flying make dexterity saving throws with disadvantage.