Information about the antrustions is derived from one of the formulae of Marculfus and from various provisions of the Salic law. Any one desiring to enter the body of Antrustions had to present himself armed at the royal palace, and there, with his hands in those of the king, take a special oath or trustis and fidelitas, in addition to the oath of fidelity sworn by every subject at the king's accession. This done, he was considered to be in truste dominica and bound to the discharge of all the services this involved. In return for these, the antrustion enjoyed certain valuable advantages, as being specially entitled to the royal assistance and protection; his wergeld is three times that of an ordinary Frank; the slayer of a Frank paid compensation of 200 solidi, that of an antrustion had to find 600. The antrustion was always of Frankish descent, and only in certain exceptional cases were Gallo-Romans admitted into the king's bodyguard. These Gallo-Romans then took the name of convivae regis, and the wergeld of 300 solidi was three times that of a homo romanus. The antrustions, belonging as they did to one body, had strictly defined duties towards one another; thus one antrustion was forbidden to bear witness against another under penalty of 15 solidi compensation.
The antrustions seem to have played an important part at the time of Clovis. It was they, apparently, who formed the army which conquered the land, an army composed chiefly of Franks, and of a few Gallo-Romans who had taken the side of Clovis. After the conquest, the role of the antrustions became less important. For each of their expeditions, the kings raised an army of citizens in which the Gallo-Romans mingled more and more with the Franks; they only kept one small permanent body which acted as their bodyguard (trustis dominica), some members of which were from time to time told off for other tasks, such as that of forming garrisons in the frontier towns. The institution seems to have disappeared during the anarchy with which the 8th century opened.
It has wrongly been held to be the origin of vassalage. Only the king had antrustions; every lord could have vassals. The antrustions were a military institution; vassalage was a social institution, the origins of which are very complex.