Thanks Raze, I might be half blind, I found it now.
Yes, I know it would be a lot of work to get this work and be fun, but I really like the idea to spice up the combat with such a feature. Larian will for sure know if it could be worth the effort, I can imagine that this might be a nightmare to optimize and it could break the balancing. Maybe it could be allowed only on Easy and Classic mode, not on the harder modes...
But for example they could also allow the guest enemy to impersonate the NPC boss only or similar, so that they could have enough power to have fun, but not enough to break the battle.
And an optional chat could be also fun, who knows?
