The following information pertains to an upcoming version (NetHack 3.7.0). If this version is now released, please verify that it is still accurate, then update the page to incorporate this information.Summon nasties can no longer create monsters of equal or higher difficulty than the caster (preventing, e.g., arch-liches or Archons from summoning more of themselves).
While Archons are formidable foes all on their own, a disproportionate number may spawn if the player has genocided several other high-level monsters; the Archon is also more liable to summon another Archon via the summon nasties spell as a result. In a worst-case scenario, this can become a chain of summons that grows exponentially and fills most of the level with Archons. This usually occurs on the Elemental Planes in the endgame, but is most commonly seen on the Plane of Fire, as it is the most open of the eligible levels; it also occasionally happens on the Plane of Air, and very rarely on other planes.
The player caught in an Archonstorm is likely doomed: even if they have a means of driving off the Archons and gaining breathing room, they are very likely to be swarmed and surrounded by the other Archons and remaining nasties. Conflict may or may not have any effect due to the Archons' monster magic resistance, and unless your AC is incredibly high, you may be struck down before you can get clear of them. As such, Archonstorms in particular are a major reason why it is discouraged to perform excessive genociding of monsters.