[Taqwus]

I don't think you can do this. While there are monsters in the game for which uniqueness is tracked (demon lords, elemental royalty, and others) and which is checked at both summoning time and map placement time (ex. if you try to put two copies of Scabiel, the Maker of Ruins in the same .map file, the first one will vanish), I don't think it's checked at recruitment; nor do I recall a way to designate a new monster type as unique.

I wonder if it'd be better down as a combination of national heroes, and independent commanders that might be 'corrupted' (have emissaries running around with 'corrupt commander' type ability), and leaving other heroic-types to be defined by the usual HoF qualifications.

Even if you took a unique-summoner and made it national (like the Tlaloques, which picks one of four specific uniques), and then modded the units... you can't prohibit the summons being invoked to bring back one who's dead, nor can you easily tie this to a particular free-for-all province instead of a nation. You could make a unique summons only castable by a rare combination for which you make a rare recruitable immobile caster, and drastically cutting gem income to make empowerment less feasible, but you can't really ensure that the immobile summoner won't be used for anything else (although if it's something truly weird like holy-6 water-1 with heavy forge and research penalties, the damage is limited, but you can only require two paths for a ritual so the number of different 'summoners' you can have is limited.)

It's an interesting period. Lots of reasons to have funky unit types running around, plenty of spies and emissaries and experts in various military matters.