Would making Hyborem (the unit) 'permanently' immortal make him too powerful? Seems like he'd be a pretty nice bolster for the Infernals if the AI (and human player) used him super-aggressively, plus not as potentially unbalancing as improving the demons/cities as there can only be one of him at once.
If you wanted to go with the score-boosting wonder, then how about a wonder in Dis ("Soulstone", anyone?
) that gives him the immortal promotion at the start of every Infernal turn? If the city is razed he loses the immortal promotion, wherever he is on the map.
So you can go rampaging with your kickass stompy demon lord, but are vulnerable to a surprise counterattack on your capital.
Dang, I should have entered that in the competition...
If you wanted to go with the score-boosting wonder, then how about a wonder in Dis ("Soulstone", anyone?

So you can go rampaging with your kickass stompy demon lord, but are vulnerable to a surprise counterattack on your capital.
Dang, I should have entered that in the competition...