How exactly does the big purple jungle monster work?

As of 26.09.2016 the Prime Orb no longer has to be carried to a specific location. The Prime Helix cards of the team that kills the boss are activated. From my experience the last hit counts, so the boss/buff may be stolen.

• Removed Orb from Prime Guardian.
  • Upon killing the Prime Guardian, Prime Cards are immediately activated for entire team who killed it.
  • No longer able to Defensive Dunk.

Source

The monster is called the Prime Helix Guardian. According to the paragon wiki, "Once your team takes down the monster it will drop a buff in which one of you or your teammates will have to carry to a targeted location to activate it. Once the buff is activated it will then proceed to activate the Prime Helix Cards in your teams deck as well as buffing your minions to make them stronger." This buff (Orb Prime buff) will, like the other buffs, last 150 seconds.

To turn it in: "there is a circle on our map(brown if the opposing team is turning in the orb, and blue if it is your team) that you must run to with the orb to turn it in. It is normally up on a platform with an invisibility pad right under it. Once you run with the orb to that spot, your team can get the Prime buff for your team. However, be careful, because the enemy team can kill the player with the prime orb and steal it. Once stolen, they can turn it in as well to give their a buff." -source