Bully

It was oh so very convenient. The logic was sound and there was nothing wrong with it in a perfectly neutral sort of sense. Taking Little Blackstar with them would definitely guarantee that it would be in Leonel's best interest not to lie to them, after all, he cared about his beast companion surviving, right? It was clever. 

However, it was from that point that everything else collapsed. 

Leonel owed these people nothing. Why would he leave his little brother with three individuals he had just as little trust in as they did him? On top of that, just because he hadn't said anything about it, didn't mean that Leonel didn't notice their shift in demeanor when learning that Blackstar was a Shadow Sovereign. 

Just how convenient was this? They got to force Leonel to hand over someone dear to him, and in their minds, he would likely die seeking revenge and then they would get a convenient and very valuable beast to use or sell in the future.