They were know for their highly zealotous Slavic paganism, and they rejected Christianity fiercly. They usually raided Christian ships from present-day Italy, as they were Slavic pirates, warriors and raiders. Today their culture is assimilated into Croatian culture after their Christianization, so they practically as a separate tribe don’t exist anymore thought a lot of present day Dalmatians has their ancestry. They were among the last Slavic people that accepted the Christianity on these territories. Were they of Croatian or Serbian origin it is unknown, most likely they where a separate Slavic tribe that settled on it’s own during the migration periods thought today it’s considered they were Croats, or at least today they are Croats, Dalmatians to be more specific.
This was an early Slavic tribe that settled Dalmatia, Red Croatia.
Pagania (the Narentine Frontier), (Greek: (Ν)αρεντάνοι – (N)arentanoi, Croatian: Paganija, Krajinjane, Neretljani, Italian: Narentani, Narentini, Serbian: Паганија, Paganija; Неретљани, Neretljani), was a land settled by the Slav tribe known as the Narentines (Neretljani) in an area of southern Dalmatia (modern day Republic of Croatia), west of the river Neretva (Narenta). They were known for their piracy.
Pagania bordered Kingdom of Croatia to the west and spread eastwards from the river of Cetina. It was consisted out of three lesser Principalities: Makar, Rastik (from Imotski to Ljubuški) and Dalen (which is not on the coast). It also included the islands of Mljet, Korčula, Brač and Hvar. The main Narentine cities were Vrulja (Gornja Brela), Mokro, Ostrog (near modern Zaostrog) and Lavćen (Gradac) or Slavineca; but by far the most important was the fortified Omiš in the later ages. At the east, Neretvia reached the river of Neretva, which faced Zachlumia.
The Narentines (Neretljani) were named by their land, Neretvia, which was named by its neighbouring river. Their secondary name, Pagans (and thus their land Pagania), the Romano-Byzantines gave them because they weren’t Christianized. The Latin name Merania, meaning the Coastland, was also used – and it gave the name for its inhabitants: Marians.
The region has also be referred to as being part of Red Croatia, in the Chronicle of the Priest of Duklja written 1298-1300 by a Catholic monk. They were also referred to as Neretljani Croatians.
The Adriatic coastland that the Narentians (Neretljani) inhabited ranged 75 kilometers from the mouth of Neretva in the Adriatic Sea at the east to the river Cetina at the west. It was 10-20 kilometers wide. Most of the soil was composed out of forests and swamps, while there was very little arable land. Also, They possessed these islands: Brač, Hvar, Mljet, Korčula, Vis and Lastovo.
The Pagans fiercely opposed Christianity, seeing it as a means of oppression, and cherished the Slavic Paganism for long.
Next to sailing, the Marians were professed in trading, growing olives, figs and vineyards. On the outskirts of Biokovo and Mosor and on the islands, they herded cattle and they were also tilling the fields to an extent. In the later medieval ages, Merania imported wheat and exported wine, solted fish, dry fruits, etc.
Their best expertise and the main income of the Pagans was piracy. The loot was split traditionally just as the catch of fish – one half goes to the provider of the ships and/or the organizer of the hunt (Prince or Archont) and the other half is split amongst the crewmembers. They were especially notorious for their slave-trade  The main type of vessel the Narentines used was the Slavic Sagena (Latin: “Sagitta”, meaning: “arrow”) from the beginning of the 9th century, a variation of the Scandinavian Viking Drakkar. It was a long, relatively shallow vessel that was specific for its high speed with a slender body, a sharp bow and a mast. It was manned by 40 crewmembers that were at the same time professional fighters. Other than this type of vessel, the Pagans used the Kondura; a ship similar to the Sagena, but a lot smaller, with a crew of 20 members. The Marians also used other types of vessels and barges.
The Marian rulers were called Morstiks and Judges, although the Church refers to them as Kings. The titles of the nobility were typical Slavic titles: Prince, Treasure-keeper, Chaplain, Knight.