This incredibly simple and reliable trap is among the safest and fastest ways to kill the ender dragon. This works only in Java. This technique was featured on Dream's Manhunt series (Speedrunner vs 5 hunters finale):
   • Minecraft Speedrunner VS 5 Hunters FI...  

* I've tested in 1.16+. A report has been made that it works in as early as 1.14.2.
** An invisibility potion may be used (without wearing armor) to minimize any chance that the dragon will attack while you are setting up the trap. This is a more important factor in 1.18+, as the dragon AI seems to be more aggressive than in previous versions, and you may become injured if standing too close to the dragon while it is in the trap.

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0:00 Intro
1:18 Materials (195 dirt blocks, 1 obsidian, 1 water bucket, pickaxe, bow, 80 arrows, optional empty bottles)
2:28 Build instructions (Pillar up to Y145, bridge to place obsidian at X0 Z0, remove last dirt block in bridge, place water on top)
4:30 Capture and kill
5:30 Demonstration on punching the ender dragon to death

The bow can be enchanted with Punch 1 or 2 without disrupting the trap.

If you use this trap to repeatedly kill a respawned dragon, you may need to block off any water flowing down from the trap so that the end crystals can be placed on the exit portal, or just pick up the water from the top of the trap after the dragon dies. Also, you may wish to invest in building a bubble column out of end stone to get you to Y145, so that you don't have to rely on pillaring or ladders, which may be destroyed by the dragon if it flies through.