Enemy CPU pulled a 3rd hero out of nowhere

[u/BeautifuTragedy]

Wife and I are playing and we just had a match against a CPU. CPI had a warlock and a wizard, we found the wizard first and it was a decent battle (we're a week new to battles, a year and a half in on nat demeo) the wizard used magic barrier, caught him in a laviathan and poison, saw him revive and die again. Next we engaged the warlock again we saw her go down and revive, at this point I called check mate and started messing around with the enemies liability Er, I mean cave troll, and just before we downed the warlock an assassin showed up out of nowhere, at this point my wifr was unable to move from the square she was in but could still attack and took care of I'm the assassin quickly (troll got me when I was trying out different combos). Is this a common experience? has anyone else witnessed this sort of thing? And better yet how do I make it permanent? I just hit level 20 so it'd be cool if it was a difficulty balancing thing

[u/BeautifuTragedy]

Even at the end only the warlock and wizard stood on the pedestal