The Tahm Kench rework had been announced for a long time, but the Riot team had postponed it until patch 11.13. The changes are mainly focused on [W] and [R].

Base stats:

Health: 600⇒570
Health Regen: 100⇒95
Mana Regen: 40⇒50
Armor: 47⇒42
Magic Resist: 32.1⇒32
[Passive]: Damage bonus: 4% of maximum health> 2.5% of maximum health.

[Q]:

* New *: Tahm Kench can now cast [R] while using [Q] to eat the stunned champion. It also heals him for 6/7 * 8/9/10% of his missing health when he hits an enemy champion and applies his [Passive] to them.
Applicable Slow: 30/40/50/60/70% ⇒40%
Duration: 3 seconds slow to 2s.
Cost: 50 mana⇒50 / 46/42/38/34.
* New *: Reach and lug scale with the size of Tahm Kench.
[W]:

* New *: Tahm Kench channels the ability for 1.35s. and then dives for 0.15 seconds into the target area. Doing so deals 100/135/170/205/240 (+ 100% AP) damage and knocks enemies into the air for 1s. Hitting enemies returns 30% of their cooldown and mana cost.
Tahm Kench will be able to take his allies with him although they will be able to come out of the River Demon's mouth while he is channeling the ability. While enemies will not be able to see where Tahm Kench will come out until 0.75 after channeling.
Cooldown: 24/23/22/21 / 20s ⇒ 21/20/19/18 / 17s.
Cost: 60/75/90/105 mana.
[AND]:

Shield Duration: 2s> 2.5s.
Gray Heal: Increases slightly by 10% per quarter of a second.
Time before gray heal: 2.5 seconds if no damage> 4s. if it doesn't take damage.
[R]:

* New *: Tahm Kench swallows the target champion for 3 seconds.
If he is an opponent, he must have 3 stacks of his [Passive] and receives 100/250/400 + 15% (+ 5% per 100 AP) of maximum health as magic damage. Tahm becomes pinned and receives a 40% speed slow.
If he is an ally, he grants a shield of 300/450/600 (+100 AP) that lasts 2.5s after the [R] ends. Tahm remains pinned and will also receive a 40/25/10% movement speed slow but can still use [W]. Allies will be able to see when [W] is active on their health bar but enemies will not see it.