We had an event similar to this in native, on nditions - called Guards and Assassins.
It worked really well, but from experience, you need a few rules.
Firstly, the king has to be escorted from the spawn to another location. It could work with a siege type mode, with a flag that the king and his bodyguard have to reach. Otherwise the king's side will camp.
Secondly, you need gear restriction for the two teams. The king needs to stand out of course. The assassins should have weaker gear, but more team members. It's also better if the gear is fitting - so assassins look like assassins (black hoods, robes etc), and Guards look like royal bodyguards (same armour, halberd type polearms, same shields). One option is to make the king ride a horse, and have his bodyguard on foot. This gives the king a chance to make a break for it if his guards are losing, and gives the assassins more tactical considerations (they have to prepare more than one ambush in case he runs).
It makes it more challenging and fun to have the assassins with lots of teammates but inferior gear, and the king with fewer guards but better gear. The guards have to work together to protect the king (shielders have to shield the king and other guards from projectiles, they have to stick together etc), while the assassins can't rely too heavily on their greater numbers because of their inferior gear - they have to find a way to make the numbers count. With the gear limit, you don't want to make it like native (too restrictive), but if you have no gear limit it'll end up as too easy for the assassins (they only have to kill one target to win).
Also, the assassins should spawn before the king and his guards, to give them time to prepare ambushes and traps.
You would have to make an entire new game mode for this to work, basically. You can do it with the existing game modes, but it needs everyone to co-operate, and the server to be passworded so no-one comes in late and doesn't know the rules.
It would also be much better with specific maps - ones with a spawn and destination point for the king, and several possible routes. Ruins works very well for this, but a more specific map would be better.