Features
This plugin makes the Elder Guardian an actual boss like the Wither and Ender Dragon. You sill have to find the Elder Guardians in the underwater temples. But now they have more health, a boss bar showing their health and name, they drop a randomly enchanted prismarine sword or amour (along with their normal loot), you can change the loot dropped to anything you want. They can also spawn regular guardians. If they hit you with their beams you get potion effects, by default poison and nausea. All the features are configurable.

Source Code Here:
https://bitbucket.org/Eliminator/elder-guardian-boss/src/master/

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Commands
/egb reload <- Reloads Config
/egb spawn <- Spawns an Elder Guardian Boss
/egb loot <- Drops Loot You Get.

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Permissions
Permission to use the reload and spawn commands: egb.cmds

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Configuration/Installation
1. Drop the plugin into your plugins folder and restart the server.

2. Edit the config to change anything you want.

3. Do /egb reload

Here is an example config: