Hive Mind is the Faceless 3,3 upgrade. It increases the production of all your buildings based on how long you have been allied with Faceless. ("Time spent", indicated at the bottom of the Stats page.)
It is also the only upgrade using a variable that persists throughout the entire game, that does not get reset at Abdications nor Reincarnations (as in v2).
Having a Hive Mind bonus of 1890% is the requirement for the Faceless Tier 4 challenge. The reward for completing the challenge is to have 5% of your Hive Mind bonus apply at all times. For this you need a Total playtime with the Faceless faction of 8 days, 13 hours, 13 minutes and 32 seconds.
Formula: where x is your total time spent as Faceless in seconds, which persists through reincarnations.