The most powerful rituals affect large portions of the world. These rituals are known as global enchantments. Once a global enchantment is cast it will continue to affect the world each game turn, until someone dispels it or the caster dies.
Global enchantments are cast on the strategic portion of the game and they cost great amounts of gems and require strong skill in magic. For each global enchantment, there is a minimum gems requirement. If more gems is used, it's harder to dispel the global enchantement.
As we can see in the following list, Mother Oak, Gifth of Health or Burden of Time are probably the global enchantements in the beginning of the game. They are only level 5 spells.
Alteration 5 : Mother Oak N5 7 : Sea of Ice W6 8 : Fata Morgana A7 9 : Utterdark D9
Blood 7 : Astral Corruption B6S6 8 : The Looming Hell B7
Conjuration 8 : Well of Misery D6 8 : Guardians of the Deep W6 9 : The Kindly Ones F6N4 9 : Wild Hunt N6 9 : Enchanted Forest N7
Construction 7 : Forge of the Ancients E5 9 : Mechanical Militia E5
Enchantement 5 : The Eyes of God S5 5 : Gift of Health N5 6 : Riches from Beneath E5 6 : Eternal Pyre F6 7 : Stellar Focus S5 7 : Earth Blood Deep Well E6 7 : Ghost Ship Armada W4D3 8 : Haunted Forest N5D1 8 : Wrath of the Sea W5 9 : Arcane Nexus S8 9 : Gift of Nature's Bounty N7 9 : Thetis Blessing W5
Evocation 6 : Perpetual Storm A5 6 : The Wrath of God S5A3 8 : Second Sun F8 8 : Maelstrom W6 9 : Strands of Arcane Power S7
Thaumaturgie 5 : Burden of Time D5 6 : Foul Air D5A1 7 : Dark Skies A5 7 : Purgatory F6 7 : Lure of the Deep W6 8 : Gale Gate A5