There has been much controversy over the exact meaning of (Mars) Halamardus. A number of proposals have been made over the years since the votive inscriptions near Roermond and Lottum, Holland were studies in the early 19th century and 20th centuries respectively. I will save the reader all the interpretations and focus on the theory which has gained most wide-spread support:
OFrk *helit, “hero, man” from PGmc *haliþaz “hero”, believed to be from PIE *kalet “hero”.
OFrk *-morth, “murder, concealment of a killing” from PGmc *murþą “death, killing, murder”. The PGmc form itself from the PIE *mr̥tós, meaning “dead, mortal”.
Inscription at Roermond:
“To the holy Mars Halamardus. Titus Domitius Vindex, Centurion of the XX. Legion Valeria Victrix. He has fulfilled his vow, willingly, as it should”
Inscription at Lottum:
“To Mars Halamardus by Titus … the Legion…He has fulfilled his vow, willingly, as it should”
These inscriptions date from the Julio-Claudian period at the limits of the Roman Empire, this side of the Rhine. Based upon the name of the dedicator Titus Domitius Vindex, which is an unusual cognomen, we can theorize that he was of Batavian origin and that the cult of Mars Halamardus (Helimerthus) was of personal (perhaps ethnic) importance to him. The 20th Legion of the Valeria Victrix soon after this dedication made incursions into Britain and so, as with other “provincial” cults of Mars, perhaps spread in various forms beyond the Batavian homeland.
The 20th Legion of the Valeria Victrix has enjoyed a vast number of inscriptions dedicated to them by various members. The cults venerated, to name a few, by those of the legion were: Apollo, Diana, Fortuna, Victoria, Hercules and Silvanus. Each of these divinities were also worshiped in provincial forms throughout traditional Frankish territory.
It is important to note that the Batavians were summarily and perhaps forcibly conjoined to the growing Frankish confederacy in the 3-4th century when migrating Frankish tribes, such as the Sali and perhaps Chamavi were pushed into their territory.
In TFA, based upon this reconstruction and interpretation of the name, we understand this god as being “He who conceals heroes upon death”. In this interpretation, coupled by the function of the epithet of Mars, we come to view this divinity as being ever-present on the battle field, surveying the struggle between factions and when the time comes, stealing away the most heroic. Where these heroes or Helithos are taken to is unknown. It is conjectured that this must be a place that is hidden and separate from the likes of the every-day warrior.
The question remains… why does he conceal the heroes? Perhaps the answer to this can be found in the custom of retrieving notable warriors off of the battle field to spare them from the humiliation of being looted or suffer indignities at the hands of the enemy. In this way, the hero is taken from the company of the other dead and given a funeral, when funerals were unlikely for the common soldier. The hero would have a chance at being welcomed by the gods, whereas the commoner would share a common resting place. In this interpretation, Helimerthus is a psychopomp specifically charged with the securing, welcoming and elevating of heroes who died in the field. Perhaps Vindex was hopeful to either be retrieved from the field upon death – or – had vowed to retrieve his commander from the field had he fallen in the fray.