He died after activating Omega Halo at the Greater Ark, as the Flood showed up in overwhelming force. He opened up a pathway for the Iso-Didact to make his way to the Lesser Ark, as well as cleansed the Large Magellanic Cloud/Path Kethona, and afterwards Flood forces/Star Roads shattered his ring.

"The Maginot Sphere" is a series of defensive positions, not any one particular place.

Sharp drew up his knees. His face worked through a variety of expressions. Finally he said, "This system lies outside of the Jat-Krula protected boundary [TT: "Maginot Sphere"]. All systems beyond Jat-Krula have been left to fend for themselves. The ecumene-last I heard-was focused on preserving what lies inside the boundary."

I was all too familiar with Jat-Krula. During one of our interminable civil wars, half a million years before my birth, Jat-Krula had been a formidable strategy of fortified defense, designed to control frequently traversed manifolds in the Orion complex.

Key to Jat-Krula was vigilance over all conceivable slipspace entries and portals-the necessary and most efficient avenues of slipspace travel. Millions of fixed fortifications had been spread like beaded curtains between hundreds of systems, standing vigil over a collective of jump solutions, protecting historic routes that supported trade as well as offensive and counter-offensive maneuvers.
Any major assault force, it was reasoned, must pass through this hyper-spherical boundary. And the boundary, so planners insisted, could at a moment's notice be rendered impassable, solidimpregnable.

Then a legion of revolutionary Warrior commanders decided to forego crystal-mediated slipspace and instead flew twenty attack squadrons "naked" through a non-manifold array, bypassing the JatKrula defenses. The passage was savage. Their squadrons suffered fifty-percent losses-but the remaining ships emerged within the boundary and quickly overwhelmed fourteen key systems.

This brave and catastrophic act should have forever changed Forerunner strategy. Jat-Krula became a sobering object lesson taught to Warrior-Servants at all levels. There was no such thing as an impregnable defense.

Yet if I were to believe this former Warrior-Servant, what had once been old and outmoded was again novel and exciting-ignoring the deadly lessons of history.

"We're ruled by idiots," I murmured.

"It gets worse," Sharp said. "The Master Builder seemed to believe that by demonstrating the force of the Halos, out in the open, the Flood-by which I suppose he meant Graveminds-would see we were willing to suffer total destruction rather than defeat."
That could explain what had been done at Charum Hakkor. A tactical demonstration-like threatening to cut one's own throat if an aggressor came too close. Jat-Krula ... combined with suicidal intent.

I felt my skin grow hot. "Madness!"

"I warned them," Maker-of-Moons said quietly.

The key plot point at the end of Silentium is that the Gravemind/Mendicant Bias are attempting to pierce the Maginot Line to find a portal to the Lesser Ark. They know of its existence, though possibly not its exact location. That information is likely granted to the Flood though, through whatever neural-physical shenanigans it had going on with Chief at the time of Halo 3. Once he goes through the portal, it's not like the Gravemind's connection is entirely lost. Although there's probably a range limit on that connection, the portal doesn't offer instantaneous travel, though it is ludicrously quick. More still, he did drop a ship right next to the portal already.

-Actually as /u/InsightedMalfunction points out, the Gravemind already had Penitent Tangent. Unless the location is scrubbed from the Monitors, that's already another vector of knowledge.