This is how you trigger the Heroic Public Event on Mercury in Destiny 2 Curse of Osiris
Destiny 2's Curse of Osiris DLC comes with a new area on Mercury, and a new Public Event to play. Just like other Public Events in the game, this one could be made Heroic to earn some more loot.
Start the event like normal. Kill Vex until the Keeper of Ages spawn, then kill it and grab its orbs. Dunk the orbs in both corresponding portals, and repeat the process and head across the area to the second Keeper of Ages.
Here's where it changes. Once that Keeper of Ages spawns, you'll see a floating prism in the air behind where he spawned. Shoot at it and destroy it. Once you do, it will disappear and platforms will spawn.
Follow the platforms up and continue shooting all of the prisms that appear. Once you're finished, you will end up on the top of the area with a Vex plate. Stand on the plate for a few seconds, and Heroic will trigger.
Now the Gate Lord boss will spawn. Head over to it and fight it like normal, except now it will have shields. To take down the shields, you must kill the Gatekeeper enemies that spawn. Once you do, they drop orbs. Dunk the orbs in the portal at the top of the area, and the shields will drop.
Rinse and repeat the process until the Gate Lord is done, and enjoy the rewards that will spawn in two chests in the area.