The one thing warlocks taught them. Blink, seriously now in Destiny one us warlocks had an entire armour set explaining how hunters could blink. What happened? Discuss.
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They didn't magically forget. The ability to blink was part of the light hunters received from the traveler. Since your light now comes from the traveler's shard, which has come to have its own splintered existence, sentience, and light, your abilities are now different.