All writers have a corrosion tolerance value (侵蝕) of 100. This may be considered the hit-points of your writer.
This value is fixed and does not change even if your writer levels up or his statistics are upgraded. Upgrading statistics affecting defense and evasion will help your writer maintain a high corrosion value.
Corrosion value will decrease when your writer suffers attacks in battle. When corrosion level reaches a certain threshold, a writer may enter a weakened or breakdown state.
Weakened and Breakdown State
In battle, if your writer's corrosion value falls to a certain point (+/- 10/100), they will go into a "weakened" (耗弱) state, indicated by a blue symbol with the accompanying letters (耗弱) above the corrosion value.
Subsequently, a "breakdown" (喪失) state, indicated by a purple symbol, may occur if your writer continues to incur corrosion during the battle.
They are not able to attack as normal in these states, although they may continue to attempt an attack during their turn.
However, in these states, they may still execute a desperation attack (筆殺奥義), which deals very high damage to the enemy Taints.
The corrosion level at which a writer will have a breakdown depends on their state of mind stat, which is fixed. The more unstable their state of mind is, the more quickly they will have a breakdown. You may view and contribute to the data on the maximum HP they will enter weakened and breakdown stats here.
Epiphany and Dual Attack
Epiphanies (降臨) between writers will occur when both of their epiphany gauges (located below their corrosion value) fills all the way up. When it is full, it will turn a bright orange color and begin to pulse.
Every hit a writer suffers will fill the epiphany gauge, but writers with a more unstable state of mind statistic will fill their epiphany gauge faster.
When two writers' gauges are filled to a maximum, they will execute a dual attack (双筆神髄).
The dual attack will launch a consecutive string of up to 3 hits culminating in high damage being dealt on all enemies present. After the dual attack is completed, both writers' epiphany gauges will be completely emptied.
If one or more of your writers are in breakdown state by the end of a battle, the game may fail your delve and send you back to the library, or it may allow you to continue, but with a warning.
This warning states that your writer's soul may come to their final words (絶筆) in the next battle if they are corroded past zero corrosion value.
Please be sure to heed this warning and return to the library without advancing to the next node if you do not want to lose your writer.
If you do advance and your writer comes to their final words, you will have to transmigrate them again and cultivate their soul from scratch.
You may protect against final words using a Philosopher's Stone. If you have one, you can equip it on the party Formation page using the silver button on each writer's panel. When in battle, you will see the stone next to the writer's portrait. If you allow your writer to be corroded to 0 HP, they will utter their final words, but after that, the stone will shatter and they will recover to 100 corrosion tolerance value.
Note: If you take the writer you are using as an assistant into a tainted book delve and they die, then your assistant will be replaced by the next writer you own, based on ID.
Another note: You may send a team including a writer in breakdown status on a tainted book delve if you click the stamp on the warning message they give you before proceeding.