This storyline was expanded upon in the
Legions of Fire Trilogy of novels by Peter David, which are canon.
So,
B5 ends with Centauri Prime basically having been conquered on the QT by the Drakh, who are controlling Londo via a 'Keeper' a parasitic extension of the Drakh overseer Shiv'kala. The Drakh are playing a long game, amassing their own forces over the next fifteen years and having the Centauri rebuild, in great secret, their own war machine. Londo seals himself off from most of his friends and colleagues in an effort to protect them from possible Drakh reprisals. This is why he has Vir assigned as ambassador to B5.
In the meantime the Drakh attempt to use a Shadow Death Cloud to attack Earth, are defeated, and instead poison the planet's atmosphere. The events of
Crusade take place and the plague is cured within three years or so (we don't find out how, though). The technomages start investigating the Drakh and learn that something is amiss on Centauri Prime. They recruit Vir to aid them and Vir eventually learns of the Drakhs' presence. He creates a resistance group called the Legions of Fire to resist the Drakh infiltration. He also learns that Londo has ordered the building of a vast warfleet which will apparently be used to launch sneak attacks on Earth and Minbar and weaken them ahead of a full Drakh assault. At some point G'Kar also learns part of this and Londo has him arrested and placed under house arrest. News of this doesn't get out and, oddly, G'Kar finds himself enjoying the solitude away from the adulation of the Narn populace. G'Kar is imprisoned on Centauri Prime for something like 2-3 years, in which time he and Londo's frendship is solidified.
Eventually, on David's 16th birthday (by Minbari reckoning; JMS mucked up his timeline and didn't realise that David would only be 14 by Earth years
) he opens the urn given to his parents by Londo, is possessed by a Keeper, and goes to Centauri Prime with Sheridan and Delenn in pursuit. The plan is for Londo to kill Sheridan and Delenn and plunge the Interstellar Alliance into chaos as just the moment the Centauri and Drakh fleets make their move.
Unfortunately for the Drakh, the whole plan falls apart. The Legions of Fire expose the Drakh's presence on the planet. The Centauri Prime Minister, who is another tool of the Drakh, is killed and all hell breaks loose. Sheridan, Delenn and David escape from Centauri Prime and Londo and G'Kar kill one another, as seen in
War Without End, Part 2. Shiv'kala detonates the fusion bombs he'd hidden across Centauri Prime (the ones mentioned in the Season 5 episode
The Fall of Centauri Prime), but the Legions of Fire have already disabled 90% of them. A couple do explode just outside the capital, flattening most of it and leaving the rest in flames (as seen in
War Without End). The new Centauri fleet attacks the Drakh fleet hiding in Centauri space, destroying most of them and sending the remnants fleeing. Vir sets himself up as bait, with Shiv'kala planning to take him over with a Keeper and start the whole process over from scratch, but Garibaldi instead kills Shiv'kala. When he dies the Keepers linked to him - including the one on David - die as well, and David is freed from the Drakh influence.
Phew. Think that about covers it
