Unlocks class when you acquire the Forlorn's Noose at the southern Seer forest. I love the Forlorn; basically a glass cannon build where you one-shot, or get one-shotted.

Tips:

Once you unlock and use the class, bum-rush the noose. Then run around collecting gold chests; you won't die to random encounters, only to sprited ones. Once around 500-1k, buy the one and only club at the Clown shop south of Rockridge. Then buy 20 or so health potions at the sympathetic shop keep at Seaside.

Save at the military fort beside Nexus forest, then find the arena trainer inside the fort, just a couple steps from where you saved. Always start with "Scared" as the first option; your feeling decides how many enemies you're fighting. The second option decides enemy level and difficulty, start from the weakest then slowly change your 2nd option difficulty. Always save after every 2-3 wins, because you can get bad RNG. Most of your fighting and level grinding will be decided by RNG.

Forlorn maximum level is 99, unlocks an OP skill but takes 50 TP. 

If you get "confused" debuffed, you will one-shot yourself, so I recommend buying that specific protection ring at the Seaside accessory shop.

Don't waste your time and energy at the farm "Old trainer."

Attack the hidden demon lord in the Southern Nexus forest, near the beach and tent, only if you're level 99 and have 800 health or more, because one dark magic attack can cut half of your health, and there's three of them. Save, die, load, die, load, die, load, until you RNG succeed and survive their first wave attacks to drop your lvl. 99 Forlorn skill on them. The club it dropped is not worth changing, only for quest fulfillment.