And let's not get into monsters summoning other monsters. A D&D 3 Marilith has a 20% chance of summoning another Marilith and making the fight twice as hard. How many mariliths do you account for when figuring how tough the fight will be? (XP is easy - summoned stuff don't give XP when killed.)
This is interesting for any edition, but D&D4 focuses on balance, and having Schrödinger's encounter be potentially twice as hard as planned is outside the design paradigm for 4E.
Fortunately, D&D4 provides examples of summoning monsters, though it's just one and a half in the whole Monster Manual. The half is the Berbalang, which summons a duplicate of itself by spend one-quarter of its HP. More interesting is the Pit Fiend, who has an encounter power that summons either 8 legion devils, 2 war devils, or half of each. However, they're weak compared to the Pit Fiend himself - if he's level appropriate, the summons are cannon fodder. Which is probably as it should be.
I had another idea. Either make the summoner a solo and let it summon minions as a recharging power (the summoner should probably be a solo), or give the summoner a higher XP cost to make up for a one-time summoning power. (The Pit Fiend does not do this - it follows the guidelines for statting an elite almost perfectly.)
If I was going to make a Marut Blademaster (Level 21) with the ability to summon his debtors, I'd double his XP value (from 3200 to 6400) and give him a minor action encounter power to summon a set bunch of monsters worth 3200 XP. Say, give him these choices:
- 4 Angel of Valor Legionnaires (L21 minions - how did these get in the Marut's debt?)
- 4 Legion Devil Legionnaires (L21 minions - our Marut gets around.)
- 1 Ghaele of Winter (L21 standard monster - okay, he outsmarted a noble Eladrin?)
There you go. Two monsters masked as one dude summoning another. Quadrupling the XP budget could obviously make one weak caster with the ability to summon up three times his own worth in opponents once the PC:s barge in.