You and your eidolon share a link that seems to span multiple lifetimes.
Benefits: Your eidolon treats its Constitution score as if it were 2 points higher for the purposes of determining when its negative hit point total sends it back to its home plane. In addition, your eidolon gains a +1 trait bonus on Will saves against enchantment spells and effects.
Pathfinder Roleplaying Game: Ultimate Campaign. © 2013, Paizo Publishing, LLC; Authors: Jesse Benner, Benjamin Bruck, Jason Bulmahn, Ryan Costello, Adam Daigle, Matt Goetz, Tim Hitchcock, James Jacobs, Ryan Macklin, Colin McComb, Jason Nelson, Richard Pett, Stephen Radney-MacFarland, Patrick Renie, Sean K Reynolds, F. Wesley Schneider, James L. Sutter, Russ Taylor, and Stephen Townshend.