LevelChangeEvent

This event is fired when the player's level count is changed. If the amount is positive the levels are being added. If they are negative, levels are being removed. This event takes place before the levels are changed. This allows you to change the amount of levels, or cancel the change entirely.

The event is cancelable.

If the event is canceled, the xp level will not change

The event does not have a result.

It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import at the very top of the file.

ZenScript
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import crafttweaker.api.event.entity.player.xp.LevelChangeEvent;

LevelChangeEvent extends PlayerXpEvent. That means all methods available in PlayerXpEvent are also available in LevelChangeEvent

ZenScript
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// LevelChangeEvent.getLevels() as int

event.getLevels();

ZenScript
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// LevelChangeEvent.setLevels(levels as int) as void

event.setLevels(5);