Stellaris: How To Defeat The Prethoryn Swarm

thegamer.com:

The Prethoryn Scourge are one of the Endgame Crises that can emerge to threaten all life in the galaxy during a Stellaris campaign. This unending swarm of ravenous bugs won't stop until it's devoured everything in its path, and it will take a concerted effort by the galaxy's mightiest powers to defeat them.

Related: Stellaris: Biological Ascension Guide

Players get more warning when the Prethoryn are on the way than they do for other Crises, so it's important to take advantage of that extra time to get ready. The Scourge can devour entire empires from the moment they arrive - don't let yours be one of them.

If the Prethoryn Scourge is coming to the galaxy, you'll receive your first warning as early as the Endgame Year set at the beginning of the campaign (2400 by default). In most campaigns the first warning signs won't appear until about 2530 at the earliest, but it is possible for them to show up sooner if you're unlucky. The Endgame Crises are mutually exclusive, so the Prethoryn won't show up if the galaxy comes under attack by the Contingency or the Unbidden.

When the Swarm is first detected on its way to the galaxy, every empire will receive a notification. The Prethoryn take an average of about fifty years to arrive after that point, but their actual arrival date is random and can be much sooner or later. When the Scourge is twelve years away, the Situation Log will mark the edge of the map from which they will enter. If you haven't started getting ready by that point, it's essential that you drop whatever you're doing and prepare for the invasion. If you're in their initial path, expect the worst.

When the first warning signs appear, dedicate your research to upgrading laser weapons, strike craft, and point