Pagan Reagents were items that appeared in Ultima Online as part of an early design for the Necromancy skill, which was scrapped for years. When Necromancy finally did appear in the game, it was very different, and the old necromancy reagents were now rare items, commonly referred to as Pagan Reagents (after the game they appeared in, Ultima VIII: Pagan).
The fiction is that these reagents relate to forms of magic no longer practiced. Whether this is "official" fiction or player fiction is not entirely clear. What is known is that they were originally used by three different sects of magic (not just Necromancy) on a planet far from Sosaria.
Regardless, the Pagan Reagents are on most shards considered very rare and valuable house decorations.
The Necromancy-based casting system in Pagan involved placing the reagents in a bag, then waving a special wand over it while chanting the Words of Power. The spells primarily dealt with the ability to prevent death, deal with those who had succumbed to it, or manipulate the element of earth.
These reagents were based on the explosive and daemonic natures of the element of fire. To cast spells in the manner of sorcery was quite complex: Both reagents and specially colored candles needed to be placed precisely around a pentagram, and then a talisman placed in the middle before the Words of Power were spoken. Once cast, the talisman received multiple spell charges that could be brought forth at any time.
The closest form of magic Pagan had to that of traditional Magery supposedly borrowed from all other forms, and so (unlike the other four disciplines) relied on no specific element - Instead drawing all power directly from the ether. The spells were cast onto the books that detailed them, charging them for later use.
Serpent's Isle Type
Not strictly of the "Pagan" variety, these three reagents were instead seen in the second installment of Ultima VII: The Serpent's Isle. They were incorporated into the standard spell casting system (which was much the same as Magery is seen in Ultima Online) for use with some of the more extravagant spells.