The Bat Bat is a magic melee weapon. It leeches 6.8 of an enemy's health with every time it is hit. The Bat Bat requires 5 Batilisk Wings, 2 Living Logs, and one Purple Gem to craft. It drains 3.4 sanity per hit while the wielder's health is not full. With a damage of 42 (63 as Wolfgang), it is the third strongest weapon in the game.
Its leeching ability makes it ideal for fighting low damage dealing mobs such as Spiders and Hounds while wearing a Log Suit, as one hit on you can be healed by one hit on the enemy. It can also be a way to easily regain lost health.
If you are low on health, a quick way to get it back without risk of death is to attack Chester. Chesters health should regenerate fast enough before he dies, although if he does he will just return 1 day later.