Regret may refer to the following:
- High Prophet of Regret- The Covenant leader.
- Regret (Level)- The level in Halo 2.
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Speculation
The message intercepted by Cortana during the first Earth Invasion was a repeating message saying; "regret, regret, regret", possibly referring to that the Prophet of Regret jumped to Earth, not knowing it was inhabited by Humans and didn't have sufficent forces to deafet them.