Twelve Truths

The Twelve Truths are the rehalian equivalent of the World Symphony - a vanguard legions of a foreign religion that happened to be close enough to the Grand Empire of Karadia when it was shattered that it happened to have an access to the Lost Lands, and through that, the Grand Empire.
The Twelve Truths's goal are the regional weakening of the imperial influence in order to help further expand the Rehalian Union. Of course, if it was the Union that would be doing the attacks, the result would be an outbreak of war between the Union and the Grand Empire, that's not something that they would be content with. Instead, when there is a need to actually attack the Empire, it is done by the angels of the One God. Such attacks are rare, are always focused on genuine military targets placed in relatively proximity of the Rehalian Union borders, and are continuously explained by the Union as retaliation for the supernatural creatures of imperial gods (the bad ones, but sometimes also good ones) attacking the border territories of the Union. Alternatively they attack the provinces where there is an on-going persecution of local rehalians, in which case the Rehalian Union rarely bothers to explain it any way.
The problem is that the Twelve Truths's attacks are typically overwhelming to an extreme degree. The rehalianism, like other monotheisms, goes for quantity instead of quality. A single angel is a terrifyingly powerful entity, and such attacks tend to come out of nowhere, meaning that it's unlikely for the attacked place to have security enough to be able to stop it. As a result, the Grand Empire tends to do its best to ignore the stings, especially as the forces of the Twelve Truths are a rare case of a force hostile to the Grand Empire that despite its hostile status doesn't attack civilian population, nor does it try to occupy anything.

Structure

Each of the Twelve Truths has its own angels and archangels, with some differences between them depending on the Truths they serve. They see absolutely no problems with cooperating, so it's extremely likely for a stronger raid target to be hit by just the right combination of angels for it to be quickly overwhelmed. Very little is known about One God servants above the level of archangel, as they are described as being too powerful to be able to directly manifest in Reality without damaging it (and as being more busy with a massively greater undertakings than an oversight of a single world), and are typically left out of the picture.
The strongest theoretically existing entities would be Truths themselves, who are often described as entities that personify their truth. If they can manifest in Reality, they have never felt the need to.

Military

Angels are the messengers, caretakers and warriors of the One God, that are occasionally send down to assist the believers. They are winger humanoid capable of wielding miracles of extreme power, with an assault of about ten angels being considered by the Grand Empire to be strong enough to overwhelm an entire praesidial province. They are regularly visiting the Grand Empire, typically as either flames of furious vengeance against the groups such as the Pentagram or Breathless Tide that dared to invade rehalian union or a much calmer strikeforces supposed to teach the imperials a lesson.
Said lesson typically ends with angels thoroughly obliterating some larger adventurer' or Imperial Army excursion, an Imperial Magic Guild research outpost or simply rolling into the province, leveling a walls of a city or damaging the local fabricae/collegium, and then retreating. This is both a form of proselytism (it is quite enlightening to see a daemon of your religion being annihilated instantly by one of another), a reminder to the Grand Empire that the Rehalian Union isn't to be trifled with, to weaken the potential bases for the assault on the Union, and as retaliation for regular attacks of the creature from the imperial cosmology on the rehalian borderlands.
It has to be mentioned, that the miracles of angels (and archangels) is quite literally a guided weapon against evil. An archangel can use an attack so powerful that it envelops an entire city, but those that are reasonably innocent (children and decent, hard working adults) and did not accumulate enough serious sins, will remain unaffected or almost unaffected by the attack. However such an attack will wipe out all local evil-doers. There are actually cases of imperial towns attacked by an archangel that ended up benefitting from such an attack in long-term, especially if they were suffering from problems of corruption and high criminal activity. Of courses, the inhabitants of such towns tend to be thankful for the angels for that, and the risk of a rehalian community developing in the area will significantly rise.