Grasp of the Undying
Grasp is AMAZING for short trades, and also gives health, both of which are things that TK loves. I don't think I can recommend taking any other keystone, but I'll still go over a couple others.
Demolish
Demolish is amazing for taking tower plates, and getting that second tower later if you're ahead. I usually take Demolish, but if you prefer Shield Bash that is fine too in harder matchups. I don't think Font of Life is worth it for top Kench.
Second Wind
Second Wind is really nice, as it synergizes with Doran's Shield and Tahm Kench E, giving you a lot of lane sustain, especially against poke champions. Conditioning may be okay in matchups that you don't wanna interact with at all early, and plan on building resistances. Bone Plating might work in matchups where they can burst half your health instantly with a combo early. I think in general I just prefer Second Wind though, it feels nice on TK as he loves snowballing health.
Overgrowth
Overgrowth has a lot of value for TK, since health is such a nice stat on him. Also, interesting fact, if you stand near voidmites while they all die, it'll give you overgrowth stacks for each one that just passively dies on its own. I'm not sure whether you'll really get time to just do this randomly in game, but sometimes if you're over there for a skirmish that didn't end up happening or something, you can Q the voidgrubs and walk away, gaining free stacks.
Triumph
Triumph is great, as it snowballs TK's health. Think about this: You W into an enemy team and get a kill, and it gives you health from Triumph. This health means you also have more time to Q and E, thus snowballing your health further. Many times this is super important to have in teamfights. I think there's a chance Absorb Life is good too, honestly, but I haven't tried it much at all.
Legend: Alacrity
Alacrity is blessed. Faster attack speed means you stack your passive faster, and you also don't have to spend as long to hit a minion, which makes your laning safer too. Along with the other two Legend runes, you can get one stack per voidgrub you help kill. So if you help kill 3 Grubs with Legend: Alacrity, you will instantly obtain 112.5 gold worth of attack speed.
Last Stand
I haven't done enough testing with Coup de Grace, Cut Down, or Last Stand, but I don't think any of these are bad choices. Last Stand will let you do more damage while you're low, and TK can snowball his HP quite a bit with Q and E. Coup de Grace I imagine is good in matchups where you wanna be able to finish their healthbar easier, such as Warwick, Olaf, or Kled. Cut Down I imagine is a tank killing item, but I'm not sure if it is good or not.
Biscuit Delivery
I usually go Alacrity and Triumph, but if you go Inspiration tree, Biscuits can be nice, as they give you sustain and allow you to fight longer while you're low health.
Magical Footwear
Free boots. I sometimes take this if I feel I need Biscuit Delivery or Approach Velocity.
Approach Velocity
Approach Velocity is really nice in matchups where you need it to catch someone after hitting Q on them. I take it against Singed sometimes, and I can only imagine it's good against champions like Teemo and Kennen too. Think champions that can gain some distance on you.
Phase Rush
I think Phase Rush is more of a fun build than anything honestly. Some say it is good against champions like Darius, but I think you can actually just short trade and Q him, and then back up. You don't typically wanna stay long enough to proc Phase Rush, just to end up doing less damage. I do think it is very fun if you're going a kidnapping build, and just run people from the middle of lane into your tower. Keep in mind also that you'll have to take 3 sub-optimal minor runes.
Fleet Footwork
Fleet Footwork is interesting. It's said to be good in ranged matchups, but I never really tried it much. I can't imagine prefering it over Resolve, as all 4 slots provide a lot of value in Resolve, and I think you can usually get at least some Grasp procs after first back even in ranged matchups. Maybe I'll try this sometime, but I just personally don't find myself super convinced yet!
Minor Runes
I pretty much always take AS shard and 2 scaling health shards. They all have a lot of value on Kench. Movespeed and Tenacity interests me, but I doubt they're typically better than the scaling health shards. At level 7 you'll already have more from your scaling health rune than the regular health rune, so I would pretty much never take the +65.
