One of these guys betrayed the kingdom and started helping the spade kingdom in opening the gates of sephiroth which was using this guy as a catalyst

For it meant that golden dawn was doing the ritual(since captain and rouge member were involved it counted them as a whole), the one getting summoned was Diablo

But Diablo is serving Rimuru who is a demon lord with the race demon slime, so while saving Diablo, He himself got summoned. As for why to Earth, it's because rimuru's world and Earth are interconnected so Earth was given priority something like:

• Priority 1: Earth
• Priority 2: Any other world

Just so happened that another group called Golden dawn

Was doing a summoning to summon rimuru as well... Since he was being summoned in the first place, he got sent to Earth instead.


(Credit to @N1s41 for this next part)
As for why golden dawn was trying to summon Rimuru in the first place, @megumer ,who was also a double spy infiltrating the government found some shady business in Area 51 and acted as a Liason between the two to dispose of it