When the Daelaam came to Slayn to aid Alarak in fighting Ma'lash, phasesmith Karax detected solarite signals coming from the elementals, and advised Artanis and his forces kill them to retrieve it. Karax ultimately discovered the elementals had an unusual digestive system, and the solarite was a byproduct of it. The retrieved solarite was used to help power the Spear of Adun.
While it doesn't appear much in pure stats, the Slayn elemental is a dangerous enemy. It can ensnare units in a solarite cocoon, preventing them from moving and attacking, and draining HP from them rapidly to heal itself. Move out in large numbers against it, as ensnared units can be attacked to free them from their trappings, also stopping the elemental from draining their health. In smaller numbers, the elemental will likely encase all the player's units, rendering them helpless until death while the elemental will likely escape the fight unharmed.
Each elemental killed gives +5 solarite.