were massing
Past progressive conjugation of mass. There are other translations for this conjugation.

mass

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In the valley below, an army of undead and goblins were massing.
En el valle que había debajo, un ejército de no-muertos y de goblins se estaba reuniendo.
The rebels were massing at their secret headquarters.
Los rebeldes se estaban concentrando en su cuartel general secreto.
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