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An Overlord is a Ruler that is not a Royal.

[edit] Proposed Canon

Known Overlords include:

• Stanley the Tool of Gobwin Knob
• Charlie of Charlescomm

[edit] Speculation

Sides run by Overlords rather than Royals can't pop Royal or Noble units. Given that neither Stanley or Charlie seems to pop Warlords, it may be that non-Royal sides are unable to pop Warlords, and must rely on capture or promotion.