Finding a Dormant Relic: Which Containers Actually Carry One
A 
Dormant Relic has no recipe. It is one of Simply Swords' unique weapons, and unique weapons reach a player through loot rather than a crafting grid, which is why searching a recipe book for one turns up nothing at all.
Which containers can carry it
The mod attaches its unique weapons to a short and very specific list of loot sources, and that list is far more ordinary than most players assume. Sixteen of the eighteen entries are village chests: the armorer, butcher, cartographer, fisher, fletcher, mason, shepherd, tannery, toolsmith and weaponsmith job sites, the plains, desert, savanna, snowy and taiga house chests, and the village temple. The remaining two are ruined portal chests and the Ender Dragon.
That list is worth reading twice, because it inverts the usual instinct. There is no ancient city entry. No nether fortress entry. No end city, no stronghold, no bastion. A player who walks past villages on the way to a deep dark raid is walking past almost every place a relic can appear.
Villages also change the shape of the search in a way that works in your favour. They generate in numbers, they sit on the surface, and one settlement holds several of the listed chests at once, so sweeping three or four villages covers more of the list than a long expedition to a single rare structure ever will. Bring a bed, clear the settlement methodically, and move on.
What you are actually rolling for
Simply Swords ships fifty-eight unique weapons and forty of them can turn up in that loot. Every chest that rolls a unique rolls across the whole pool, so one named blade is a small slice of a result that is already uncommon. Expect to meet several other uniques before this one.
The Dormant Relic is one of three that sit apart from the rest of the pool, alongside the 
Decaying Relic and the 
Enigma. Their in-game tooltips point at carrying them rather than swinging them. The Dormant Relic reads that a faint power seems to emanate from within, and a later state describes that power as growing instead of faint. The Decaying Relic is blunter still, telling the holder to keep it in their inventory while exploring the deepest and darkest depths of the Overworld.
Once one is in your inventory
The 
Runic Forge is the block the mod points at next, and its in-game description is short and exact: it is used to awaken unique weapons. The 
Runic Tablet carries a line of its own about repairing and upgrading uniques, and it sits in the reroll recipes as the template that pairs a runic weapon with a diamond.
When several villages come up empty
A server can change every part of this. The loot list is configurable and includes a switch that turns unique weapon loot off entirely, so the eighteen entries above describe the mod as it ships, not necessarily the world you are standing in. After a few thoroughly looted settlements with nothing to show, ask whoever runs the server what the loot settings are before blaming your luck.