Tahm Kench isn't underpowered. [DISCUSSION]
Tahm Kench has the lowest winrate in Ranked Solo, with a 37% winrate, the average being 50%. However, despite his winrate, Tahm Kench isn't underpowered at all. He has great abilities, and does take some good skill.
The 1 major reason why he has such a low winrate, is because, he has nothing to help his team with. During teamfights, he mostly just sits there, and devours a teammate, and walks away.
What should really happen, is Tahm Kench, if he swallows a minion - he should get a special attack from devouring that type of minion.
It'll overall let Tahm Kench have much more viability, and be a lot more interesting.
Like, if Tahm Kench devours a Super Minion, he will shoot it out, and it will push enemies back and deal damage.
If he swallows a melee minion, it should deal X of a target's max health when he shoots it back out.
Something like that, to actually help his team, so that his winrate goes up. He has basically no mobility, and sits there in a teamfight.
He has a GREAT laning phase - however, he falls off really hard, and desperately needs some changes.