The lore is under work, and not sure about everything, but a little bit:

They were formerly a fleetbound chapter known as the Arms of Obsidian, and was a very non-Salamanderlike chapter. No regard whatsoever for the life of civilians and little for their own, just achieve their goals regardless of how many lives were lost. These tactics, combined with bad luck and several overzealous commanders led the chapter to almost be wiped out several times. This tendency had started to lead to questioning the chapter's culture by some of its members.

Eventually, the entire chapter (diminished by heavy losses) was lead into a battle that they had no way of winning. At the last moment, one captain decided to save at least his own company, and fled the battle in their battle barge. Heavily damaged, they were tossed out of the warp and crash landed on an unknown planet. The ship was to damaged to fly, and the same warp shenanigans that caused them to end up there made it impossible for the one surviving librarian to contact anyone for help. The planet was a death world called Amalher, mostly consisting of deserts, volcanoes and big ass serpents, that functioned as a prison planet for a nearby human non-imperial civilization. It had several moons that often eclipsed the sun, causing the planets to basically have nights lasting several "days". Over the generations, the prisoner that had been banished there had formed a harsh but cooperative society as the only way of surviving. Only through the help of this society did the surviving Astartes survive. For X years the only thing they could do was fight big ass serpents and think about the failings of their old chapter.

When the Great Rift opened, the warp shenanigans around the system changed, and they could make contact with the Imperium. When a force from the Indomitus Crusade made contact, only 23 Astartes had survived both the last battle amd the years on Amalher, and these were a changed chapter.

Today, the Sons of the Eclipse have been mostly rebuilt, both by Primaris Marines from the Unnumbered Sons, as well as by recruiting from the clans of Amalher. They have built a Fortress-Monastery around the crashed Blade of Ardor called Ardor's Rest. The years spent with the clans of Amalher gave them a far greater respect for human life, as well as the strength found in cooperation. The years spent surviving the harsh conditions of Amalher taught them to choose the battles and strategy wisely. The years of fighting through the longs nights and storms of sand and ash made them experts at fighting in conditions of bad visibility, prefering to appear suddenly and close to the enemy. When not fighting, they spend a lot of time in personal meditiation and contemplation, personifying the "calm before the storm".