Cool, but unlikely to kill Tiamat

It's health is massive, and so is its Armor Class, but Tiamat has an average attack roll of 29, which would allow her to hit the Ethereal Warden. It has a mythic action which gives creatures disadvantage on attack rolls against. That would lower Tiamat's average attack roll down to 24, which would miss the Warden. But the way mythic actions are supposed to work is that they activate when the creature is reduced to 0 hit points. They have a feature that then puts their hit points almost back up to max, and unlocks mythic actions, which are just extra choices for legendary actions, and are used the same way as a legendary action. So if used correctly, it wouldn't be able to use the disadvantage feature until after knocked down to 0 hp. 

The Warden does have spellcasting that allows it to cast up to 2nd level spells and also cast cantrips as legendary actions, but Tiamat is immune to all spells of 6th level or lower, as I have already mentioned. Its main action is True Sword, which allows it to gain advantage on its next attack roll and then make a Warden Blade attack. Warden Blade only does 20 damage per round though, and Tiamat regains 30 hit points at the start of each of her turns.

The Warden is immune to nonmagical physical attacks, but Tiamat has magic weapons, and bypasses that immunity. But you did give the Warden resistance to magical physical attacks, which would allow to take half damage from all of Tiamat's attacks. But Tiamat's breath weapons would just shred this thing's health. It does have high enough Dex and Con saves to take half damage from her breath weapons. Tiamat's average damage per round to the Warden would be 83 damage. The Warden's average damage per round is 20, and Tiamat regains 30 hit points every turn, meaning the Warden deals no damage to Tiamat.

It would Tiamat 26 rounds to kill the Warden, which isn't enough time for it to use its imprisonment ability.

Ok, but TiamatsWarrior, I intentionally made its armor class 28, So tiamat could hit it. Also, this is technically a god, and could just use its teleport ability to survive for 100 rounds.

Its teleport ability would teleport it 200 ft. away from Tiamat, but Tiamat has a 120 ft. flying speed and 90 ft. cone breath weapons, which would allow her to reach a target up to 210 ft. away.

This is actually pretty balanced though, and except for the mythic actions, this is a very good stat block

There are a few things that make it unbalanced, such as the health, but everything is at least slightly balanced

But its low damage output balances the high health.

And tiamatswarrior, it could teleport, and then use 200 ft. of fly movement to get 400 ft. away, and stall the battle for 100 turns.

Actually, only 24 rounds.

And it being a god, I think would allow it to bypass the Need to die, but sure, lets change it a little.

It could actually get away from Tiamat, but then by doing so it would end the combat. And when it comes back 100 rounds later, it would be the start of a new battle.

Quote from TiamatsWarrior >>

Cool, but unlikely to kill Tiamat

It's health is massive, and so is its Armor Class, but Tiamat has an average attack roll of 29, which would allow her to hit the Ethereal Warden. It has a mythic action which gives creatures disadvantage on attack rolls against. That would lower Tiamat's average attack roll down to 24, which would miss the Warden. But the way mythic actions are supposed to work is that they activate when the creature is reduced to 0 hit points. They have a feature that then puts their hit points almost back up to max, and unlocks mythic actions, which are just extra choices for legendary actions, and are used the same way as a legendary action. So if used correctly, it wouldn't be able to use the disadvantage feature until after knocked down to 0 hp. 

The Warden does have spellcasting that allows it to cast up to 2nd level spells and also cast cantrips as legendary actions, but Tiamat is immune to all spells of 6th level or lower, as I have already mentioned. Its main action is True Sword, which allows it to gain advantage on its next attack roll and then make a Warden Blade attack. Warden Blade only does 20 damage per round though, and Tiamat regains 30 hit points at the start of each of her turns.

The Warden is immune to nonmagical physical attacks, but Tiamat has magic weapons, and bypasses that immunity. But you did give the Warden resistance to magical physical attacks, which would allow to take half damage from all of Tiamat's attacks. But Tiamat's breath weapons would just shred this thing's health. It does have high enough Dex and Con saves to take half damage from her breath weapons. Tiamat's average damage per round to the Warden would be 83 damage. The Warden's average damage per round is 20, and Tiamat regains 30 hit points every turn, meaning the Warden deals no damage to Tiamat.

It would Tiamat 26 rounds to kill the Warden, which isn't enough time for it to use its imprisonment ability.

 Would this work as a final boss for a campaign?

If played right, this could be a very good final boss

Advantage on all attacks, and can give disadvantage to its enemies