Lots of events are actually parts of overarching chains of events. In order to prevent people from ever "losing" an event, many events have a "Win" path and a "Lose" path, some resulting in mini-bosses or larger bosses depending on outcome.
I recently have been running around the Norn areas and one set of events involves needing to prevent some kind of ritual, then if that fails they build some kind of totem, followed by the opening of a set of portals to summon ice elementals, and then finally a Svanir Shaman shows up and creates a fairly large scale battle and lots of blizzard effects over a lake of ice. The overall key to spotting bosses is knowing which events lead to which other events in sequence and to know that a boss is at the end.
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