From the publisher:
The Tidal Lord reigns supreme while standing in Water regions on the map. Enemies sharing its region suffer 10 damage each turn, while Allies seeking the Tidal Lord's protection recover 10 health each turn while in its region.
Like the non-elite Fen Lord, the Tidal Lord can slow down enemy units. The Riptide attack reduces enemy movement by 3 for one round, but if it shares a Water region with the Tidal Lord this duration is 2 rounds.
The Tidal Lord can also utilize a Pull attack like its non-elite brother -- but the Churning Sea has the option of targeting Allies, in which case it gains Accuracy and does zero damage. Combined with the Tidal Lord's passive ability, it can either pull enemy Golems to their watery doom, or re-position allies for healing.