How we turned the doom vault dungeon in to a cake walk

So our campaign starts like any other D&D game does big bad end of the world etc. etc. the party at the time consists  Kitsune Bard rouge, an Aarakocra monk rouge, a Human cleric fighter, a Loxadon Barbarian, and a Tabaxi rouge I joined later as the dwarf paladin (( yes I know a lot of rouges )) at the point we are all fairly leveled I think about 12th at the time our group and DM are pretty chill about things we are going threw the doom vault after we where teleported their by force to find a disturbance with the balance of elements it all goes … well (( besides our monk setting off EVERY trap how he has survived this long I will never know )) we come to these torture chambers with shackled demons and chained monsters as a paladin I'm fine leaving them there but my party had other plans we kill the cultists' torturers and our bard wants to set this demon free and without hesitation or bargain begins to unlock its shackles hoping by its good graces that we killed its captors would help us in the dungeon. It did not go as planned as one of the manacles are removed our monk walked into the demons radius and is captured our bard casts a spell and it does nothing, with its arms free the demon casts a spell and almost one-shots the bard 5hp and paralyzed (( our DM later told us it was a challenge rating 15 )) I quickly cure her status effect while the other rouges undo the demons shackles so it doesn't kill the monk my only thought is crap we are all dead but with enough luck (( and a really good roll )) our bard casts suggestion on the demon and tells it to leave and it just walks away. after that, the cleric and I heal the party and move on to the next section not without giving our bard and monk some crap and a little praise to the bard for the save.  we all move on to the next part and come to a cross-section on where to go next 3  paths the first leads to trap (( of course )) to the right the demon is standing there in front of a door and in front of us in a large room that smells like a farm I go right to check the door slipping past the demon and not wanting to set off a trap knock on the door only for it to fling open and inside is a group of hell guards and just around the corner a lich. the party closed the door behind me (( because they're funny )) I panicked and said dungeon keeping our DM had me roll a performance check (( did I say she was chill no shes evil )) I did not roll well but it was so funny the lich let me clean the room long enough for the party to open the door and let me out after that  the lich followed me out  to come face to face with a very large demon attacked it and we walked away. 

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