Fully built up conduits can kill hostile mobs in a certain radius as long as they're in water.

If a mob leaves that radius, they will no longer take damage from the conduit. However, if they just leave the water without leaving the radius as well, they'll continue to take damage and eventually die.

To reproduce

1. Place any mob inside water directly next to a fully powered conduit
2. Wait for the conduit to attack once
3. Remove the water or move the mob out of the water
   → The conduit continues to attack and kills the mob

Videos

• Shulker: Minecraft 1.16.5 - Singleplayer 2021-06-03 17-31-22.mp4
• Skeleton: Minecraft 1.17 Pre-release 4 - Singleplayer 2021-06-03 17-35-14.mp4