So. We have a lot to unwrap here.
First, what do we want? What do our party members want? Clara and John are probably both happy to serve under us, but Matty's motives are still a mystery to us. Currently, our objective is Tributescar. Eventually, we may wish to get Matty a fancy new sword, seeing as his old one got vaporized: he may know a better location for that than us, supposing he's an angel. That, or we could just ask Clara. As for us, however, I'd like to see if we can't ultimately get our cool goblin chick into the party. Past that who knows, but we've got stuff to do for now, so.
Next, some things to consider.
1. In the arena is a dead forgotten beast with deadly blood.
2. We have blood powers.
3. We have used said blood powers to make a laser pointer before.
If'n you see where I'm going with this, we have the option to make an incredibly lethal laser pointer should we be able to collect any of its blood. We may want a new getup if we're gonna choose to work with it, though, since it would be cruel irony to get killed by the stuff twice. It might be a nice idea to eventually get a flask or two that we can store blood in (one for beast blood, one for normal blood), since being able to make a handy blood-something sounds like quite the useful ability. Still, that's a later thing, unless we find a way to get some beast blood.
These objectives in mind, assassinating the king seems like a great idea: it should cause some chaos, maybe allow us to get into the arena for blood-getting and goblin-rescuing reasons, and then get outta dodge and see if we can't find our resistance buddies. We should be aware there's probably a significant portion of the fortress military nearby, and also that we're literally unarmed, unarmored pincushions/sword racks for any soldiers that happen by: even if we have immense hemomantic power, it's only any good on wounded creatures, and matty's punching can only do so much. As long as we're in here we can use the beast blood laser pointer to wound the enemy (just like, shine it in their eyes) and then just exsanguinate them from there. Problem, however: if we kill the king, the soldiers might just execute our goblin gal on the spot so they don't have to deal with keeping an eye on her and dealing with us.
Kill the king, then heroically jump into the arena to save goblin gal, exiting stage right (through whatever door we can get into). Don't forget to make sure both ourself and Matty get some weapons; whatever's laying around in the arena will do for now.