Ekko is most often used as a mid lane assasin. That works just fine but the problem is you can get destroyed if another lane is fed and once your ult is out you have no protection. Tank ekko has very good sustain and damage. You obviously now rely more on your R for damage but since its scaling is insane and you're still building AP you now just have more HP.

For me, personally, the most important reason is that with the directions Riot has taken with its Patches, it becomes harder and harder to solo carry with assasins like Kha'Zix and also unfortunately Ekk. Riot makes it very clear that League of Legends is a team game and whilst it might still be possible to solo carry in lower elos, especially in gold and above its not so easy anymore. Tanks dominate the meta in my opinion and thats why I opted to play ttank Ekko

Pros:

- a lot of sustain
- good early trading
- still massive damage from R
- no fed lane can destroy your game like with assasin ekko
- insane healing

Cons:

- not as much carry potential
- when ult isn't up you have no good team fighting

and

Grasp of the Undying is super strong. its basically the go-to for every tank and the only thing that really makes sense for this ekko build on the resolve tree.
Shield Bash resonates well with your W shield. If you want you can take Demolish for more splitpush power. Bone Plating is pretty self explanitory since its the best in its branch and gives good sustain.

The second tree is Precision. You can also always go for Domination with Sudden Impact and Ravenous Hunter. I think that Triumph and Coup de Grace are great for dueling and are especially crucial in the early game.

For the Rune stats you can take two times Adaptive Force for that extra AD, but be sure to take Health. If you feel unsafe you can swap one of the Adaptive Force for Magic Resistance and Armor, as usual dependant on the match-up.

Items

Dark Seal is probably not a good option. Its still viable but as tank ekko you're probably not gonna get the most use out of this item. Also Doran's Ring give some early HP which is (obviously) good on tank ekko

Rod of Ages should always be part of your core items. It gives MASSIVE with its passive and also a lot of AP. Sunfire Aegis and Spirit Visage are so strong in the current meta that depending on the match-up ( Sunfire Aegis if your lane opponent is mainly AD, and Spirit Visage against an AP Match-up) you should atleast build one. Spirit Visage's passive also resonate with the rune Revitalize and your Ult which then heals for even more.

You have to evaluate. I can't dictate how your game is and what to build. If you're ahead an want to build some ap go for any of the ap items. If you're struggling with Crits build Randuin's Omen. Against Zed always build Zhonya's Hourglass and if youre struggling with mages build either Adaptive Helm or Abyssal Mask. Zz'Rot Portal and Gargoyle Stoneplate is against a bit of everything.

In early game play like a normal assasin Ekko. Go for trading and killing minions with your Q and be sure to abuse your Shield Bash after using your W. Try to finish your Rod of Ages at 10 minutes because you need those 10 stacks as quickly as possible. If you aren't able to finish it because you're very much behind just go full tank instantly and don't go for items like Rabadon's Deathcap etc.

In Late to Mid Game be their front line at all time. You can protect your ADC's but be sure to tell that you R because you don't want your ADC's not being prepared for losing their tank for a few seconds.

IF you took Demolish an important part of your late game will be split-pushing and forcing their team to split to fight you. Just be sure that if there isn't a second tank try to be in almost every Team Fight.

Always with your ultimate you have to position yourself in a way that you can use your R effectively.

That's basically almost everything to look out for. I can't tell what your match will like. Like always in League: [Adapt. Improvise. Overcome.