Apparently there's a glitch in Darkest Light that prevents it to convert negative to positive if you don't know the Twilight talent (whether this is a bug or not isn't really relevant since it only really "works" that way for Adventurers, which are best described as a broken exploit-abusing class anyway
So how can we exploit this glitch to get permanent invisibility? Simple: get Necrosis, ascend to Lichdom at 24, get Eclipse, side with the Assassin Lord. The rest is up to you.
1) Works best on spellcasters, because Corona is a prerequisite for Darkest Light. If you want Corona, you'll also want either Twilight Surge (1 point is enough) or Celestial/Light or it'll never trigger.
2) If you rely on Twilight Surge rather than Light for positive energy generation, AND sustain a Chant (you got from a Sun Paladin escort) but no Hymn, your Darkest Light is guaranteed to never deactivate (unless you cancel it yourself.)
3) Tricks of the Trade reduces the damage penalty and unlocks (or upgrades) the Stealth tree, so you can run both invisibility and stealth at the same time - which makes you nigh undetectable past midgame by everything that hasn't invisibility detection. Plus, Shadowstrike makes up for what remains of your damage penalty.
4) Totality is also a prerequisite for Darkest Light, so it makes sense to invest in Moonlight Ray and Spell/Nightfall.
5) You can also ignore Corona, Twilight Surge, etc. and invest in Cursed/Darkness for ultimate stealthiness. Or whatever floats your boat.
I'm not saying this is a game-breaking combo (on the contrary I think it's definitely on the underpowered side), rather it's just another fun exploit to try.