I know I'm gonna be off on the lore, been a while but didn't Crota do something with the Vex that pissed Oryx off?
English
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Crota tried to be smart but accidentally opened a vex gate into oryx throne world, so basically the vex breached the one place oryx could truly be killed cause crota was all like "imma make dad proud! ......-blam!-"
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He opened a portal for them into the dreadnought
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I think he got pissed because crota let the vex into our universe