If you got a "Hell's Portal" in your Hand while playing a Demon Card, and if #Gems>13, replace one "Hell's Portal" on your Hand with a Demon Card of the Higher Rarity and reduce your Gems by 13.
If last Card played while triggering this effect is rare:
I think this is a truly unique and awesome suggestion. I think it may need a little tweaking though:
When triggered, the next card will be a demon of equal or greater rarity. Gain 1/3 of the cost of the summoned card.
I think this has a feel of bringing a demon through a portal as part of an onslaught, but there's a chance that you get very little resources.