If you go to -i believe- halo wikia. It states ‘helioskrill’ is a predator on sanghelios that waits for its prey by looking like a rock. Just thought i should clarify, i just dont get why people think its an armor 
They are referring to the armor set you could unlock in the Halo 5 Beta. It was called HELIOSKRILL, and was unlocked by getting the legendary achievement in MCC
You can unlock Helioskrill in Halo 5 multiplayer by unlocking the “Legend” achievement in Halo: MCC, which entails completing all 4 campaigns on legendary difficulty.
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> They are referring to the armor set you could unlock in the Halo 5 Beta. It was called HELIOSKRILL, and was unlocked by getting the legendary achievement in MCC
Halo wikia begs to differ
Joke thread
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> http://www.halopedia.org/HELIOSKRILL-class_Mjolnir
>
> You can unlock Helioskrill in Halo 5 multiplayer by unlocking the “Legend” achievement in Halo: MCC, which entails completing all 4 campaigns on legendary difficulty.
I think it’s a Prophet fish. And an armor.
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> > 2533274888579326;2:
> > They are referring to the armor set you could unlock in the Halo 5 Beta. It was called HELIOSKRILL, and was unlocked by getting the legendary achievement in MCC
>
>
> Halo wikia begs to differ
> Joke thread
Gottcha. Keep doing what you’re doing.
Basically a chupa-thingy.
> 2533274915983628;4:
> > 2533274888579326;2:
> > They are referring to the armor set you could unlock in the Halo 5 Beta. It was called HELIOSKRILL, and was unlocked by getting the legendary achievement in MCC
>
>
> Halo wikia begs to differ
> Joke thread
No dude, jokes are funny.
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> > 2533274843498392;3:
> > http://www.halopedia.org/HELIOSKRILL-class_Mjolnir
> >
> > You can unlock Helioskrill in Halo 5 multiplayer by unlocking the “Legend” achievement in Halo: MCC, which entails completing all 4 campaigns on legendary difficulty.
>
>
> begs to differ 
"Helioskrill, a Sangheili-designed MJOLNIR armor variant, is an unlockable armor permutation in Halo 5: Guardians. "
It’s a mistake, that’s what it is.
Whoever decided to let aliens have access to MJOLNIR designs should be convicted of treason and then spaced. They may be allies, but that’s no reason to give them such easy access to sensitive technology.
You get it from completing all four campaigns on legendary in the MCC