[Zeldor]

Arcane Nexus - if there are many players it can bring huge amount of S gems even on first turn, so even if it gets dispelled immediately, it's a great investment [hundreds of S gems a turn!]

Utterdark - killer of non-undead nations, unless someone has gold piled up for upkeep, he loses 90-100% of his normal armies [not commanders], also Darkness in every prov gives hude advantage to very few nations

Burden of Time - annihilates all human nations within few turns, they simply lose most [if not all] of their commanders due to old age

Astral Corruption - it costs just blood slaves, which are easy to get, and pretty much disables effective forging and spellcasting for non-blood nations