Relic Weapons are special fabled weapons, some of which repeatedly appear in the Final Fantasy series. Relic weapons either play a significant part in the lore for that job; or are the most powerful weapons for that job in previous Final Fantasy games. Once obtained, relic weapons can be upgraded through a series of time-consuming tasks and quests to potentially become the most powerful weapons in the game at the time.
There is a different series of Relic Weapons in each expansion. Other than at the beginning of the Anima Weapons, there is no interaction between the different expansions' relics. You can do them in any order you choose.
Relic weapons from past expansions are not intended for combat use, but for cosmetic, achievements, and collection purposes. Weapons of similar power, for the same player level, that are much more easily obtainable and are intended for character progression can be bought with Allagan Tomestones of Poetics.
Relic Weapon Series
Lv 50: Zodiac Weapons
Lv 60: Anima Weapons
Lv 70: Eurekan Weapons (See also: Eurekan Armor)
Lv 80: Resistance Weapons (See also: Resistance Armor)
Lv 90: Manderville Weapons
Relic Tools Series
Lv 80: Skysteel Tools
Lv 90: Splendorous Tools