It is the personal weapon of Brave Lucina.
Wielded by Brave Lucina, this lance passively boosts her Defense by 3 and grants Atk/Spd +3 to all allies within two spaces of her that is wielding any weapon that is not a Tome, Staff, or Breath weapon.
As of the 3.9 update of the game, the 4 first Choose Your Legends units got a refine on their weapons, which includes the Geirskögul. Any refinement add to the lance a effectiveness against dragons, and the special refine grants Def/Res +3 to said physical weapon allies within her range, and gives them a "Breath" effect, giving their special cooldown charge +1 to each strike.
Grants Def+3. Grants allies with sword, lance, axe, bow, or dagger within 2 spaces Atk/Spd+3 during combat.