A Storm Altar is an area in Definitely Not Dominion game mode.
There is only one altar at the center of the map. Both team can try to capture the altar, giving their team the Nimbus Storm buff.
The Nimbus Storm buff description:
Spend a charge on hitting an enemny champion to create a bolt of chain lightning that hits up to 3 other targets, dealing true damage equal to 4/5/6% of target's max health (half for secondary targets).
You gain one charge of Nimbus Storm every 4 second while in combat.