Rend doesn't seem to display by default, so I used:

return DebuffAliases {
772, -- Rend
772, -- Rend
}

Still with no luck. Any idea's what I'm doing wrong?

You are using the wrong IDs.

Debuff ID: 388539
Spell ID: 394062 (Prot) and 772 (Arms)

The rule you need is:

return DebuffAliases {
	{
		   772, -- Arms Rend
		394062, -- Prot Rend
	},
	388539, -- Debuff Rend
}

This is what debugging tooltip gives you btw

TYVM Kind sir.