This perk has been disabled, though not yet fully removed.
Deathwish is a perk that is available from the start of the game after choosing a hero. When you take this perk, any damage taken while you are below 35% of your maximum life will increase your auto-attack damage, spell damage, armor, and shield armor by 45% for 20 seconds. This perk is best used for tank class units that have large health pools and can continually proc the damage and extra armor and shields to continue taking the damage. Alternatively, getting the Health and Shield buff from the Cube will give all heroes a lightly better chance of sustaining this perk longer.
A good way of utilizing this perk is to keep your health at about 25-35%, take a hit, back off, and do damage. Once your 40 seconds is up, your health will have (most likely) automatically regenerated itself past 35% for you to take another hit and reproc the perk. Another way to use this is on the Brain Bug, as it emits a continual poison cloud around it, giving light but steady damage. You may opt to repeat the above, or sit in the cloud if you are able to sustain yourself for long periods of time in the gas.
At its current stage with all the passive regen and other perks, Death wish has no use for any char but tychus solo runs, and even there, Air perk is debateable better.