A couple things actually, I'm glad you asked. Though they aren't all about the character.
Firstly: Arty never raised his arm. He just left it where it was behind his back and opened it, since it was already pointed at you. There wouldn't be that much warning.
Secondly: Lith has no energy absorbing properties, and if it's still in second form it also has armor running up your arm. That's going to conduct all of the electricity straight through your body on its way to the ground. Even if you managed to plant one end in the ground and still get the other in position to block, a significant portion would still go through you.
Now on to the character.
Demonic Presence: How strong are the tentacles? And 20 meters is over 60 feet. That could hit almost all of us at once. I'd be comfortable with something less than 5 meters across. Perhaps each tentacle can generate enough spikes for one square meter, giving you 8 square meters (a 12 foot by 12 foot square) if all of them are used. I'd also like to know the range of this ability.
Demon Wings: Being as fast as a fighter jet puts him at over mach 2. Mach 1 is kind of the unofficial limit for intermediate characters, but unless he has some way to propel himself in addition to just his wings, he'd never even get close to that speed. A peregrine falcon (the fastest known winged creature) has a highest recorded speed of 242 mph, and that's in a straight dive, not level flight.
Pure Dark: I'm unclear as to what the first portion of this is supposed to do.
Unholy Sacrifice: I think this one should probably have a 2:1 enhancement ratio, given how powerful Alex already is. So if he gave up his inhuman speed, he would only gain an equivalent half of what he gave up back as strength.
Natural Adaptation: If this means he can copy anyone else's abilities, I'm going to have to say no to it. Suddenly gaining the ability to control lightning after watching Arty do it would put him way outside the bounds of OP when combined with his already very potent abilities.
Prowess of a Demon: First off, 50 tons per arm is enough to crush every single one of our characters with one blow, including Vordak's, whose only abilities are his physical power, speed, and durability. The strength is going to have to be massively nerfed if you want to keep all his other abilities, probably in the range of 1 ton baseline (total, not per arm, though he can go higher with his unholy sacrifice). Secondly, having senses so perfectly attuned that they let him avoid any attack is blatantly OP. If this is the level he's at after all his power has been lost, he soars well into high tier and way past any characters here.
Shadow Puppets: How durable are they and how many can he make at once?
Weaknesses: Just a general note about how attacks or defenses that generate a lot of light should probably weaken his shadow manipulation abilities.