Posted byon June 16, 2019.
Last updated byon February 5, 2020.
Source: Rewarded randomly when using 5 Animated Particulates received from the Talos of Gaia during the Anniversary Event. The Shem of Magma will only appear as a possible pet through Animated Particulates after having faced the Magma Talos at least once.
This Pet cannot be traded or sold on the Auction House.
Magma is probably slightly inaccurate. Shem of the semi-cooled remnants of molten stone is not nearly as poetic though.