Young Black Dragon called Gendridd, wasn't meant to be a major obstacle, his personally is that he's evil mostly just because he's having a fantastic time being an asshole and constantly taunting people (to the extent that the first and so far only lair effect he's got is the ability to heckle people at any location in the lair). Anyways he was fully aware that he'd get one-rounded trying to fight a party of four level 7 PCs, so instead of fighting them stole the party's unattended bags and sat in a tree to taunt them about it before flying off (he did not consider this might lead to fighting them anyway).

The party's plan was to ambush him in his own territory, so their plan was to cut through some of the most overgrown parts of the swamp to get behind his lair and set up an ambush, instead of confronting any of his minions. However, between several spellcaster party members who had both completely dumped their strength/dexterity and couldn't cast spells while drowning, party members wearing full heavy armour that weighed them down significantly, and bad rolls, then two of them fell into a bog, and in trying to rescue them the others also fell in and they all drowned, resulting in the first TPK.

Obviously that wasn't a super satisfying ending, so for closure I offered to run a oneshot with a level 5 party in Gendridd's lair, sent to avenge the original party, on condition that I wouldn't hold back with enemy strategy and tactics (no bullshit with magic, just good enemy postioning, balanced teams that had lots of options in fights, and had actual battle plans). They made it through most of the dungeon pretty well, while constantly trading off verbal barbs with Gendridd who basically ran a snarky sports commentary the entire way through, letting them know how eager he was to crush them when they made it to HIS big boss chamber. Anyways they reached the outside of the chamber and they were just preparing to fight the skeletons who were guarding his door when he jumped out of an acid river behind them and got a Surprise Round, hitting two of them with a breath weapon and then rolling good enough initiative to knock out their fragile backline casters.

After that then he's become popular/respected enough to get Promoted To NPC.

TLDR: First wipe due to RNGiamat cursing the d20 and a party badly suited to dealing with falling in a bog. Second wipe because Gendridd employed the secret chromatic dragon art of 'lying to people'.