Ley Lines,Nexuses, and Holy Sites
Ley lines are rivers of mgical energy that circle the world. Invisible to the untrained, (Arcane Magic Skill will allow you to see them) they flow around the world and pour life into the land.
In game terms, a ley line provides access to 100 mana per turn and is likely to have more verdant soil, more game, more beasts and be the prized turf of one humanoid or another.
A nexus is the convergence of two or more ley lines and is often also a holy site.
Nexuses will provide 1.5 times the mana the involved ley lines normally would. A three ley line nexus for example, would provide 300 mana per turn times 1.5 or 450 mana per turn. Nexuses are almost always guarded, if not by humanoids then by the flora and fauna in an area. The massive magical energies in and around nexuses often cause bizarre magical phenomena and mutations in the resident creatures.
Holy sites are massive Faith mana vaults. Favored places of the gods. Pillage the energies stored here at your own risk.
Generally speaking a holy site will be a mana vault that holds 1000 or more mana and recharges over time. If it is also on a ley line or nexus it will recharge even faster.