Well of Shadows is a dungeon added in the Definitive Edition. This dungeon contains the Lobb Shot Rifle upgrade, which you will need for:
The I Kan Lobb achievement in ReCore worth 145 pointsKill an enemy corebot with a Lobb Shot that spends 3 seconds in flight The dungeon runs you through using that mechanic, and ends with a boss fight against Shield of Midnight. For this achievement you don't need to earn all of the dungeon rewards, just complete it.