When I was patrolling with my friend in Rheasilvia, there was a text that popped up saying "A Saboteur wanders the Dreaming City". We then found an Ultra Taken Minotaur, killed it, and it dropped an orb called an "Awoken Charge". I found out that you can throw and catch this Awoken Charge just like the greed balls in Spire of Stars.
Now, I knew where this charge went, because I had previously been up to one of the rock isles on the east side of Rheasilvia, where there was a circle that "Requires an Awoken Charge". I put the charge in, and it said "A good deed is rewarded", and then it said "Phuura, Farseen Heir comes with stolen riches".
Phuura is a giant ogre in Rheasilvia, and it's shield can't be broken unless you throw powered up Awoken Charge orbs at it. The way you get more orbs is by killing the Taken Knights and Wizards next to Phuura. You charge the orbs at purple glowing spots by the bridge that Phuura spawns at. Once charged, you can throw them at Phuura to damage it's shield. Takes about 3-4 orbs to take down the shield.
Phuura also has Raid-boss level health, so you still need to use the charged Awoken orbs to kill it.
I haven't been able to kill it yet because me and my fireteam were still figuring out the mechanics to this fight.
*update: The event only seems to spawn when there's at least 3 guardians in Rheasilvia. I've been waiting for it alone for about 30 mins, it's not spawning.