Are there special rewards for the temporary Hexakill gamemode?

Solution 1:

Yes there is 1 summoner icon for Hexakill on twisted treeline (It takes up to 3 days before it can be used)

MARK OF THE BETRAYER ICON

Win three games of Hexakill: Twisted Treeline

With the harrowing event they also released some other icons

HARROWED PUPPET ICON

Give a gift of any kind to a friend before the Harrowing ends

And

SHADOW ISLES CREST ICON

Pledge your soul to the Shadow Isles for 1500 IP during the Harrowing, or 250 RP any time afterward.

Here is a link with all information about the harrowing event.