Baal Pirates can appear on turns 2 or 3 of floors 92-99 of the Land of Carnage Item World. They appear very rarely, and can take over an hour of repeating this section to make them appear.
They arrive as a group in the Carriage, and will make a beeline for your party. They have extremely high stats. I encountered them in a legendary Rag Suit (level 3 item). The regular enemies were in the 1600-2000 level range, while the Baals were level 9999 with 130 million+ HP and 6-10 million per stat. You'll want to have at least one maxed out level 40 weapon and some maxed Trapezohedrons before facing them.
Their attacks are very strong, and can take your character out in one barrage. Have the Naive Glasses (grants 100% hit chance) you won from your X-Dimension battles handy in case you've got low chances to hit. Leverage any available Geo Panels you can to get an advantage. Keep in mind that they are immune to Deathblow panels, and cannot be lifted.