You have a very nice idea but, sadly, this sounds like a character from Smite. She's a character who can shift between a light and dark form instead of having an ultimate ability. This idea is fairly different but, thanks to the passive of gaining darkness, it would mean designing two ultimate abilities(unless the ultimate itself was something that caused instant 'darkening'). No champion has two ultimates(from my recollection, I could be wrong) so, in all honesty, having the ultimate be a shift into her/his dark form would be more appropriate but, again, make her/him a very close replica of the character from Smite.
I've never created a champion, or really thought about doing so, and I have a problem, from a past experience with something different, of making completely over powered characters so I don't think I'd be good applying my knowledge here. I can see others faults much easier than I can see my own so I'm just gonna leave my own ideas on this concept and let others do the work. (I could help, if needed.)
My ideas is that her/his passive be she gains stacks of darkness(really, we don't need another manaless/energiless champion) so she/he can run off mana and gains 1 stack per unit hit, 2 for champions. At 100 stacks the transformation happens.
In her/his "light" form there should be some form of shielding/healing/disability clearing(honestly having a champ who could clear disabilities on allies without the need of 
would be pretty neat. Again, I could be mistaken and/or have forgotten someone who can do this already. I know 
works well for being saving disabled champs, but not removing the disabilities themselves) but shouldn't be allowed to be cast on his/herself if she/he is stunned or suppression or other like disabilities. While, on the opposite side of this skill it turns into a slow/blind. After all, something that clears disabilities would probably cause them if corrupted. So, to recap, light half ability should be a support skill of some kind, in my mind, whole the dark half should be a disability skill(that may or may not cause damage.)
The second ability should be an area spell that, clearly, is meant for gaining more stacks of Darkness. The spell in it's light form should be smaller, less damage dealing, and hold a stun(for follow up to dark side and next ability idea.) that lasts for 1/1.25/1.50/1.75/2 seconds. There should be a delay of some length, half a second delay minimum. Dark side of this ability would be a larger, harder hitting spell that holds the same delay but doesn't stun but, instead, erases disabilities on enemies for the added benefit of making all single spell DoT skills instantly do full Dot damage. Again, it should only be for single target spells, making it useless for satan's shrooms. Does not work with items. (A good follow up for champions like 
. So all around team supporting, here.)
The third ability, in its light form, should be a single target DoT spell that summons an orb of light that deals low damage in and of itself as long as the target stays in the area but, if let build for five seconds, it deals 10/12.5/15/17.5/20% of targets maximum life as magic/true damage. The dark side would summon an orb of darkness that does much heavier DoT without the end benefit of % damage to maximum life.
The ultimate, to negate the champ having two ults or being a semi-copy of the god from Smite, should be called Balance/Symphony in both his/her forms and deal area damage around the champion after a delay. Depending on form there would be an added benefit. Light Form: 1/3/5% of damage done is returned to him/her as healing(reduced to 33% for minions/monsters). Dark Form: Enemy champions hit receive 10/15/20% increased damage for 3 seconds. (Helping to increase the true/magic damage, to 24% at max, of her/his light form's third ability if timed correctly.)
Looking back I feel as if some of this might be OP but, at the same time, has it's own setbacks making me feel like I may be over analyzing. Again, it's just my own ideas for the abilities and might need to be worked out more. Just thought I'd throw out my two cents! Feel free to critique anything and everything here. (Feedback is what's helped me calm down on creating overpowered BS in the first place so, yea, please do.) Alter anything here if you wish. No need to quote me or give me credit. I'm just hoping this idea of Sylphyed's goes somewhere!
