Bugs. Lots of bugs, who have a Hive Mind, and can force other people, cult-like, to become Joiners, part of the Hive Mind. They try to take over Chiss territory. The Chiss violently object. The Republic gets involved, leaning slightly more towards the Chiss side. The Jedi get involved, a number of them turning out to be Joiners who think that letting bugs devour everything is a great idea. This is notable as it's the first time Ben has a personality. Written by Troy Denning, the same author who killed Anakin Solo in his part of the New Jedi Order. Gorier and more explicit with sex than most Star Wars EU books.
Asshole Victim: Jacen uses the Force to obliterate Hapan Queen Mother Ta'a Chume's mind to protect Tenel Ka and their daughter. Ta'a previously tried to sabotage the Hapes Consortium's joining the New Republic, had her Dathomiri daughter-in-law poisoned during the Yuuzhan Vong War, and then hired the Dark Nest to have Tenel Ka killed. No great loss.
Bus Crash: Chak Fel, one of Soontir Fel's sons, a pilot who gets angry about injustice and knows when to put aside his pride and who is just generally awesome, is casually mentioned to have died in a war that's now over.
Leads to a Crowning Moment of Funny when Jaina declares to Jag Fel that she still has feelings for him... and right afterwards, Jag realizes to his utter dismay that due to mind-melding, Zekk also shares these feelings. Try imagining this: your romantic rival feels for you the same way your romantic interest does. Jag's expression is understandably a bit disgusted.
Hive Caste System: There are a number of different types of Killik, many of them radically different in appearance and tactics. While each type is connected mentally, they apparently have a different Hive Mind for each type, and so when the Jedi Joiners are sent to assist a different type, they find their Joiner-ness weakening. Being Joiners, this horrifies them.
Hive Mind: Killiks, and the Joiners who get absorbed into it.
Ho Yay: Zekk, hiveminded with Jaina, agrees with her feelings towards Jag. Probably not deliberate. Probably.
Idiosyncratic Episode Naming: The first two books are The Joiner King and The Unseen Queen; the third is The Swarm War. Also all three begin with a definite article.
Squick: Luke uses the Force to make a pair of Ishi Tib find him attractive, though he's normally hideous to them. For reference, male Ishi Tib look like this◊. Then they molest him. Why Denning saw the need to give him this power is unclear.
Well, he was trying to make them (and everybody else in the room) like him more, to help with the subsequent hero-pumps-the-bar-for-information scene (like Charm Person - the spell, not the trope). It just Went Horribly Right.
Theme Naming: "Killik" is a palindrome. All Killik species have palindrome names, from Aebea to Yoggoy.
Unreliable Narrator: The Jedi who become Joiners think that the bugs are awesome and hiveminds are awesome and Chiss are meanies for not, you know, letting the bugs take over. The New Republic is also wrong, for not supporting them one hundred percent. And of course, the Killiks couldn't possibly slow down their reproductive rate and remove the exponentially increasing overpopulation problem and the constant food crises. That's the only reason why they're invading, of course.
Of course, the Chiss think the Killiks are nothing but insects and pests and they, the Chiss, are justified in killing as many of them as necessary without making more than token efforts at negotiation. Maybe we should kill them both and use their understudies?
An explanation might be required: Oroboro is the Killik name for Alderaan.
Jagged Fel is banished from Chiss society because of the damages Lowbacca inflicted while under custody.
Not so much the damage he did while in custody, but the fact that he was released under a promise that he would stay out of the fighting... and then joined Jacen in the assault on the Chiss base that started the war.
Zerg Rush: The primary Killik tactic; they're described as dying in the millions when facing entrenched enemies.