I'm convinced I have the smartest readers! Illamasqua Underworld is indeed one half of the equation, with Tantrum Sheer Lipgloss making up the other half.
Top-bottom: Underworld, Tantrum
L-R: Underworld, Tantrum
Illamasqua Underworld topped with Tantrum
Underworld is a pink-violet duochrome that goes on sheer and is almost impossible to build-up without drying out the lips. I wouldn't recommend wearing this alone unless all you want is a vague pink-violet sheen. Tantrum, on the other hand, is a pretty straightforward grape (even smells like grape!) gloss.
I didn't like Underworld till I layered Tantrum over it. What was patchy and uneven became the amazing shown above.
I'm not sure if Underworld is a limited edition product. It's not currently listed on Sephora's website. Thankfully, for those interested, Tantrum is still available for $20.