Well, here goes. I'll give it my best shot (I think)-- the Sphere of Life and The Horizon were two different magic vessels, so to speak. As you mentioned, Sloane and Nadia found and retrieved the Sphere in 3-22, 'Resurrection"--Siena, Italy--that mission shown in flashback form in The Descent, 4-20. Very confusing. As you recall, in 4-20, as Nadia touched the Sphere of Life, she saw a vision (although she had no idea what it was or meant at that time) that forecast the finale story (4-22--"Before the Flood")--Nadia ended up in a medically-induced coma (4-22--5-01--"Prophet Five"). Sloane described the Sphere of Life as intrinsic to Rambaldi's "essence," but it seemed to be an artifact deemed necessary to bring about Rambaldi's endgame by providing a vision of the future. The Sphere of Life exchanged several ownerships--Elena and her minions--then recovered by APO (4-22, "After the Flood) and as far as we know is it's still on location at DSR's HQ. Yeah, right.
Well, the story of the Horizon is seems less convulated--and involves APO's and rival Prophet Five's search for the location of the Horizon-a bowl-like vessel somewhat similar to the Sphere of Life. Sydney was kidnapped, hypnotized in a lucid state, gave up (on purpose of course) the wrong code name for an old Alliance mission that, naturally, involved the Horizon. To Paris (Jean Bertrand) and from there Sydney, Jack and Irina (who had joined them) found themselves in a Vancouver bank where the Horizon is locked away in a savings deposit vault. (Strangely enough, the package that contained the Horizon was book shaped, not bowl-shaped-hmmm, maybe the Horizon was collapsible.) It (although hinted) wasn't until "All the Time in the World" that we finally learned that it was the Horizon (along with the red ball spinning above) that yielded eternal life--its function, and why Sloane and Irina joined forces (for the last time). For Sloane--no surprise--seeking eternal life seened to have been his main focus in pursuing Rambaldi's endgame. For Irina- world power, and the riches that came with it--plus eternal life. We know that Irina died reaching for the Horizon, but we don't know if it, too, has become a museum piece at DSR's HQ. Ah well, I'll take Sydney, Vaughn, Isabelle and baby Jack, anytime Well especially Vaughn. Wish he were here. Persimmon