Yeah, I should know this by now, but it came up the other day and stumped the group. They wanted to throw a grappling hook on a knotted rope 80 feet up and snag a small window they'd broken. We were trying to figure out how much the knotted rope would help, but the best we could find was that a magic rope of climbing gives you advantage on the climb check. Fine. So I said your grappling hook knotted rope does the same thing. You climb half speed, so to get that far up would be three checks. The tricky part was setting the DC. I said 12, but maybe that was too high with the knotted rope. The fat dwarf cleric wants to climb up too and the player (my brother) wants it easy peasy it's so easy. DC 5 is he thinks is reasonable. Which, I guess back in 3.x a knotted rope does drop the DC to 5. Maybe I should have said DC 8 or 10. I wanted some difficulty of climbing up 80 feet. Failure wouldn't mean you fall, it means you don't make progress and have to make another check.
The fat dwarf had Athletics +5. The rogue only had Athletics +1. They ended up trying something else because it seemed too dangerous to climb a knotted rope that high up.
How should I have handled this better?
So, I am just replying to the OP without reading everything through, apologies if all of this has been covered...
FWIW, we rule climbing a normal rope is DC 8, a knotted rope would grant advantage at our tables.
Using your two PCs (dwarf and rogue) would yield this (with DC 8 and advantage):
Dwarf (assuming speed 25) would make three checks, needing a 3 or higher (99% with advantage), would make the climb without issue (3 successes) 97% of the time. In my book, that is easy peasy.
Rogue (assuming speed 30) would only need two checks due to Cunning Action Dash, needing a 7 or higher (91% with advantage), would make the climb without issue 82.81%. Not easy peasy since the rogue has a decent chance of not making a check (9%) but still pretty darn easy.
Failure would only risk falling if you failed by 5 or more. So the rogue falls only on a 1 or 2 (1%). I would allow a DEX save DC 10 to catch himself.
So, the dwarf
can't fall unless you allow it on double 1's, and the rogue will rarely fall and make it 99.5% of the time over all, even if he might need an extra check.
IMO, the
hard part would be getting the grappling hook into a small window 80' high!!!
