Sacrificing

Sacrifice

If your tower fails to go through the sacrificing process to be promoted to the next rarity and you choose not to retry (or you've run out of chances), it will become broken.
To retry a sacrifice, you'll need to pay gems. Every retry will cost you more gems.
To repair your tower, you'll need to spend Impure or pure souls.
The highest rarity you can currently gain from sacrificing is Celestial.
Success isn't guaranteed - even with all retries.
Higher rarity will increase the units' base damage.

The Chances of next rarity

Common > Rare = 30% chance

Rare > Legendary = 27% chance

Legendary > Mythical = 24.30% chance

Mythical > Godly = 21.87% chance

Godly > Divine = 19.68% chance

Divine > Ultimate = 17.71% chance

Ultimate > Arcane = 15.94% chance

Arcane > Celestial = 14.35% chance

Retry

If your sacrifice fails, you will have the option to retry it by paying gems. Each time you retry on a single tower, the cost will go up. Shiny units costs twice as much to retry as nonshiny units.

The prices for retrying per attempt are:

Nonshiny units:

1st attempt = 200
2nd attempt = 300
3rd attempt = 450
4th attempt = 700
5th attempt = 1050
6th attempt = 1600
7th and final attempt = 2400

Shiny units:

1st attempt = 400
2nd attempt = 600
3rd attempt = 900
4th attempt = 1400
5th attempt = 2100
6th attempt = 3200
7th and final attempt = 4800

You only have 7 attempts to retry. If you fail all 7 attempts, or you decide not to retry, your unit becomes "broken" and cannot be sacrificed again until it is repaired. For shiny units the gem price is 2 times higher.

Repairing a broken tower

Once your tower has become broken, it can be repaired by using Impure Souls or Pure Souls.

Repairing a tower with Pure Souls will guarantee it to be repaired. Only 1 Pure Soul is needed per repair, this does not depend on rarity.

How to obtain

You can obtain Souls by obtaining them as a drop from Raids or from the Battlepass if they're listed as a reward in the current season.

Repairing a tower with Impure Souls will not guarantee it to be repaired. Impure souls only have a 35% chance of being successful and also cost multiple depending on what rarity the tower that you're trying to repair is.