Hey Cryoshell. This is Osprii, you just reviewed my champion, Nahuatl.
Anyway, on with the review.
The innate is quite powerful. I realize it only gains 5% of the MR drain as AP when 5 champions are around and a full 25% with only one champion around, if I understand correctly. That is not what concerns me. What concerns me is that everyone starts with 30 mr, right? And Fiddle decreases enemy MR by 10. That's it. At level 1, that is about 33% of enemy MR, less than what Helba drains, and he doesn't gain AP from it. At level 18, 10 is waaaaaaay less than 35% of enemy MR, since melee units gain a bit and most people buy MR items. It is ok for Helba's passive to be more powerful than Fiddle's passive at level 18, as long as it is weaker at level 1. I would suggest a 10% MR drain, and gaining 50% of that as MR (from a single champion, or 10% from 5 each) to make it more balanced.
Anyway, it's a super cool idea, despite my critique on the specific balancing. Sorta like the opposite of Galio, making Helba more powerful against MR stackers, conversely.
Q: A V is an interesting shape for a basic attack modifier. Any particular reason for it? The numbers seem quite balanced to me, considering the cooldown, but the cost is high. Nasus has a basic attack modifier with comparable cooldowns and stacking, and it has a cost of 20/25/30/35/40.
W: This is also a solid ability. The AP ratio is highly pushed over to the DoT side of the ability, which may be a slight disincentive for building too much AP beyond a Rylai's and some tanky-ish AP stuff on Helba. 6 seconds is a long time, and Singed deals 210(+90% AP) over 3 seconds and Teemo deals something (+80% AP) over 4 seconds. Most of Teemo's AP is stacked over towards the immediate side of the damage, not the over time side, too. What I'm saying is that I think that 6 seconds is too long of a time, and reduces the effectiveness of this ability, making it slightly underpowered. Sure, Rylai's on this will be a *****, but the damage is so spread out. Maybe only 4 seconds? Anyway, I can't prove what I'm saying outright, so it's your choice.
E: Well, this is a lot like Cho'gath's rupture, except with a longer cooldown and less of an AP ratio. Oh, and I'm assuming it has no delay, unlike Cho'gath's, which has a .5 second delay or something like that. If there is no delay, it is ok for this to have a smaller AP ratio than Cho'gath's, because it would take less skill to hit and reward crowd control more consistently. However, this has high base damage at level 1. Cho'gath's starts at 80 and ends at 305, I think. 100 is a huge amount of damage, coupled with a knock up. It would make Helba an early game terror.
R: This is a long channel and a super powerful skill. The reward is ridiculously high, as well, with a 5 second fear. This sounds so difficult to get off that I think there might be one successful ult every other game, and probably only if Helba built tanky and the enemy had no real cc. That's underpowered. However, when Helba did get it off, the enemy would automatically die because Helba could do whatever she wanted while they all ran around for 5 seconds straight. That's overpowered. This combination of super over and underpowered parts makes the ability quite unbalanced. It's a cool sounding skill, the AoE fear on transformation makes it sound really awesome, like a cooler version of Shyvana's ult. To make this viable, my suggestion is to tone down the channel to about 1 or 2 seconds, then tone down the fear to 1/1.5/2 seconds as well as the range to 650 units, and reduce the base damage a bit while increasing the AP ratio to 100%. You can also reduce the cooldown. I guess you wanted this ability to be super powerful and super hard use, but League of Legends is about balance. Play DoTA for a little while and you'll see why. That's why everyone has damage and cooldowns and ranges in the same area, for the most part.
Anyway, solid champion, all of her abilities are conceptually sound and interesting. The numbers may need some more balancing, but I do like Helba overall. The lore is also interesting, if short, and it's cool how it connects to your other champion. Good job.