Galeos desires the power of Blade of Escata because he realizes it holds the power of Chaos. He creates a plan with Dagul Bos to retrieve the blade and the Elemental Gears. When Dagul Bos's attack on the Kingdom of Escata proves fruitless, Galeos realizes that Henri, the surviving heir to the throne, must be in possession of the blade.
Galeos hires bandits to capture Henri and retrieve the Blade of Escata. The plan fails when Henri is protected by Laura. Revising his plan after recognizing its flaws, Galeos decides to manipulate Henri into collecting the gears himself. He offers to take Henri under his protection, but Henri has grown attached to Laura and prefers to travel with her.
Galeos sends Francis, a bodyguard from Escata, to keep watch on Henri and Laura and report back to him; however, he does not tell Francis of his plan to obtain the power of the Blade of Escata.
Once Henri collect the Elemental Gears, except for the one that Dagul Bos has taken, Galeos offers to keep the Blade of Escata in his own possession for protection while Henri confronts Dagul Bos with the plan of betraying Dagul Bos by allowing Henri to kill Dagul Bos and bring back the final Elemental Gear.
If the player loses the fight to Dagul Bos, Dagul Bos betrays Galeos and take the blade and gears. Galeos no longer is an important part of the story and Dagul Bos becomes the final boss that summons Chaos with the Blade of Escata.
On the other hand, if the player wins the fight against Dagul Bos, Galeos collects the final Elemental Gear from Henri and reveals that he was manipulating Henri to collect the power of the blade. Francis abandons his loyalty to Galeos in favor of Henri.
Galeos takes the Blade of Escata to Nakle Lines to unleash the power of Chaos. When Henri shows up to stop him, he acquires the final requirement to unlock the power of the blade, a member of the bloodline of the Escata family. Galeos does not realize the full power of Chaos and is absorbed into Chaos after his fall to Henri.
As a boss, Basil's strength lies in damaging the party's HP. In fact, the majority of LP damage Laura's party will incur will come from Basil's fondness of combos. While Basil has single-target attacks, the majority of his attacks are designed to damage (and potentially Stun) many targets at once. It's best to tackle Basil using 3 characters per round; as you'll take less overall damage this way.
Basil's "Shadow Breath" attack can inflict Blackout, making Null Blackout equipment a necessity.
For all the lines of the character, go here: