And now the main question:
why do the dark subclasses only have one super?
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Around Beyond Light, Bungie was considering removing “non-IP-Defining supers” from the game and limiting all subclasses to 1 super each. This was scrapped after a huge wave of community sentiment. This was eventually scrapped, but the legacy lives on in subclasses after that point only having 1 super. This may change in the future tho. New Darkness Supers could be nice once they finish giving all the light subclasses their 4th aspects. So maybe Episode 4-ish?