Magus Vigor

 is an Arcane Enhancement for the Operator that grants them additional health upon transferring out of the Warframe.

Acquisition
The Magus Vigor is a non-reusable blueprint can be purchased from The Quills for  provided the player is at Mote rank or higher.

Health calculation
The formula is:

$$hp = (basehp + Magus ~ Vigor) * Enduring ~ Tides ~ bonus$$

therefore with a maxed Magus Vigor, if combined with the Vazarin Focus school's rank 3 or rank 4 Enduring Tides ability +150% multiplier we get

$$hp = (250 + 200) * 2.5 = 1125$$