There are a handful of events in Nightreign that will occur randomly throughout your run, and these events will make things even harder to complete the level. The Locust Swarm Event is one of the toughest events to beat, as you end up losing all your levels if you do get attacked. This guide will explain how to beat the locust event in Nightreign.
Locust Swarm Event Guide Elden Ring Nightreign

The Locust Swarm Event happens randomly during a run, and you can identify it happening when you spot a purple fog in your surrounding. Usually, these pests appear after defeating a large chunk of Nightlords, and you will get a unique objective to find the enemy stealing your runes. Keep an eye out for a black cloud of insects. When they appear, avoid their area of contact if you want to keep your levels intact. Once they leave, follow them to complete the event objective.
To locate the culprit, follow the swarm and it will lead you to the Sentient Pest Nightlord. Sometimes these swarms are easy to miss while navigating the difficult terrains, and you might have to explore the other side of the map to find the boss. That will require you to find a second swarm, so keep an eye out for that once the first swarm dissipates. Also, the Sentient Pest won’t appear as a whole entity, and will rather appear as an orb at the start of the fight.
How to Beat the Sentient Pest

Interact with the orb to reveal the Sentient Pest boss. This boss is not as mobile as the rest of the bosses in Nightreign, but it will attack you with poison gas occasionally. Besides this move, the boss will also command the growing swarm of locusts to attack, further draining levels and health. You have to keep dodging around the swarm while trying to attack the Sentient Pest. The fight is pretty linear, but you and your teammates have to be quick about downing the boss.
Dodging and getting close to the boss is the only way to complete the fight. Once the boss is dead, you will get a unique passive called Integration of Intelligence, which gives you runes every time you attack. Also, all your levels will be restored as runes, so you need to spend them all to level up your character and regain what was lost. For defeating the Sentient Pest for the first time, you will also unlock the Plague of Locust achievement.
For more Elden Ring Nightreign guides, you can check out the ones on Best Classes To Play Solo, How to Get More Flasks, and the Shifting Earth Events Explained to know more.