The Immortis Gland, also called the "God-Maker", was a genetically-engineered organ about twice the size of a thumbnail that existed within the cerebral cortex of each of the Primarchs. Created by the genius of the Emperor of Mankind in His gene-laboratories beneath the Himalazian Mountains before the onset of the Unification Wars in the 30th Millennium, the Immortis Gland was responsible for the Primarchs' gigantic size and superhuman strength. The organ also regulated each Primarch's rapid growth to maturity and his overall metabolism, granting him a lifespan measured in Terran centuries, if not functional immortality.
The Immortis Gland was structurally divided into two hemispheres or valves, the right or "Dextrophic Lobe" and the left or "Sintarius Lobe." However, after the completion of the Primarch Project and the loss of the still-gestating Primarchs to the machinations of the Ruinous Powers, all materials or genetic blueprints for the creation of the Sintarius Lobe of the super-organ were wholly eradicated from Imperial records. This was done either by the Emperor's own hand or by some nefarious, unknown source.
After the Horus Heresy, when Archamagos Dominus Belisarius Cawl was tasked by the Primarch Roboute Guilliman, the Lord Commander of the Imperium, with improving upon the Emperor's original design for the Astartes, he was able to replicate only the creation of the Immortis Gland's Dextrophic Lobe, which he renamed the Magnificat, "the Amplifier." The Magnificat was one of the three new gene-seed implant organs created for the Primaris Space Marines by Cawl, including the Sinew Coils and the Belisarian Furnace.
A small, thumbnail-sized lobe that is inserted into the cortex of the brain of every Primaris Space Marine, the Magnificat secretes hormones that increase the body's growth functions whilst also intensifying the function of its other transhuman Astartes implants, especially those of the Biscopea and the Ossmodula. This leads to the increased size, strength and reaction times of the Primaris Astartes compared to their original brethren.
- Codex Adeptus Astartes - Space Marines (8th Edition), pg. 11