Well, turns out even with closed doors, heroes never caught a whiff of Splig:
"Campaing Phase (continued)
Heading for the Finale, heroes meet a travelling sage, removing Pit Trap from the Overlord's hand, but then a host of bloated zombies at the river inflicts Disease on all the heroes. Rest of the travel is uneventful and the heroes arrive at their destination.
Quest: The Man Who Would Be King
Heroes: Tahlia (Knight), Aurim (Disciple), Jaes the Exile (Runemaster)
Encounter 1 (open group: shadow dragon)
Things have a chance to go horribly wrong for the heroes right away, as Dark Charm tries to work its way through Jaes's willpower, with the close quarters and Lightning Strike, very likely all the heroes would be affected as he'd Blast them. Thanks to the Lucky Charm however, Jaes resists the urge. Worse for the Overlord, despite using Dark Fortune, the Shadow Dragon fails to get a surge, thus no Fire Breath. Splig takes off, but without Dash can only open the door and wobble on a few more spaces, while the other two lieutenants make it towards Splig at full speed, Alric Farrow wielding the Duskblade, while Belthir has the Scorpion's Kiss.
After attacking, the Shadow Dragon backed off away from the heroes, blocking off the doorway. Aurim uses his Heroic Feat and picks up a Power Potion, then steps twice with fatigue and Searches, picking up a Warding Talisman, before wounding the Shadow Dragon with his Dwarven Firebomb, but forced to use the Power Potion to clear a miss. Jaes has no such aid and uses his Heroic Feat, only to roll a miss, but at least wounds the Shadow Dragon some more with his second attack. Tahlia engages the Shadow Dragon, but fails to roll a surge and he attack goes to waste.
Splig continues his escape, but now gets behind Belthir and Farrow, a tough pair of obstacles the heroes must get past to reach Splig and they still have the Shadow Dragon to deal with. Shadow Dragon that heals itself using Dark Resilience and attacks, but fails to deliver meaningful damage. Despite being still wounded, the heroes are forced to use up most of their attacks to bring it down, only Tahlia making it through the doorway, which quickly proves a mistake as the Shadow Dragon returns and double moves to block the doorway once again, separating Tahlia from the other two heroes, while Farrow and Belthir block the path into the Armoury as Splig reaches the penultimate door.
Under the onslaught from Aurim and Jaes, the Shadow Dragon goes down, as Tahlia wounds Belthir. However, as the lieutenants return the favor, Tahlia is knocked out, Pierce 5 from the Duskblade carving through her defenses like they weren't even there, with Belthir using the Scorpion's Kiss to Immobilize Jaes after assisting Farrow in taking down Tahlia. Aurim Revives Tahlia, then steps close to Jaes and uses the Prayer of Healing on him, getting rid of the Condition, which then allows Jaes to attack twice on the lieutenants, only to miss his first attack, before inflicting some solid pain with his second.
Tahlia keeps on sucking, failing to get through Belthir's defenses with her first attack, missing her second. Then it all goes to hell. Farrow takes down Tahlia again, bagging the Overlord several more cards from Bloodlust, followed by Belthir doing his Cry Havoc to drop Aurim and hitting Jaes for six, with the Shadow Dragon then finishing off Jaes with an hit for seven damage. With all the heroes knocked out, Splig does a Dash and exits. Overlord wins.
Encounter 2 (open group: shadow dragon)
With all of the heroes getting a free Stand Up during the transition, it leaves them each with no more than five health, which, given that knock out takes them out permanently in this encounter, isn't much at all. However, Aurim raises the hopes after using his Heroic Feat and drawing a Health Potion, using it right away to at least get health back to full for everyone. He then Rests to clear off his fatigue, while Jaes blows up the master Zombie, followed by Tahlia taking a minion Zombie, then another with her Heroic Feat. Jaes fails to resist the Dark Charm and zaps himself for five damage.
Shadow Dragon emerges from the Throne Room with a Frenzied Dash and rips into Jaes, but only wounding him slightly. But just as the heroes are preparing for their offensive, the Shadow Dragon goes into a Blood Rage and bites into Jaes, who is taken out by its first attack! Heroes are down to two. At least the Blood Rage also removed the Shadow Dragon, so the heroes rush forward and engage Baron Zachareth who has moved up a bit too eagerly. Aurim hits him for four, while Tahlia gets only a single wound after Dark Fortitude surrounds the baron.
Reinforce brings back the Shadow Dragon, but before it activates, the lieutenants hack at Aurim, then back away, with Zachareth retreating all the way to the throne, while Eliza and Merick Farrow block the narrow path between the obstacles, preventing the heroes from getting into contact with the baron. Shadow Dragon then goes into a Frenzy and rushes the heroes, taking down Aurim and wounding Tahlia with its Fire Breath. Tahlia hacks at Eliza and even with the Shadow ability hindering her, manages to deal eight wounds. But this proves her last hoorah as the lieutenants and monsters converge on her, with the Shadow Dragon taking out Tahlia as well, fitting end given that it had already taken out the other two heroes as well. All hail King Zachareth as the campaign ends in an Overlord victory!"
Even if Aurim had done another attack, instead of a search on turn 2 (since heroes skip turn 1), at best that would've perhaps killed the Shadow Dragon (it has two health left when I healed it up), but would've still taken up all they hero actions, meaning the SD just spawns back and double moves to block the doorway yet again. By turn 3, which is the second turn for the heroes, even without Dash, Splig is already behind the wall of Belthir + Farrow. Granted, OL might not have them in which case things are easier, but since I was in charge to picking all the Act 2 quests, didn't want to touch either of those quests, thus they are counted as OL wins. So really, the heroes have to kill the Shadow Dragon, get LOS on Splig and Immobilize him on turn 2, Stun works if there is no Dash for the OL, but if he has, he probably plays it turn 1.
And it could've been so very different (even worse) for the heroes. Since they are all adjacent at the start, if Jaes doesn't make his willpower roll against Dark Charm, he'll Lightning Strike and very likely get Blast (via surge) and hit all the heroes for around 5-7 damage (before defense). Additionally, even with Dark Fortune, the SD couldn't get a surge, if it had, that's another seven dmg attack on all the heroes thanks to Fire Breath. Not to mention Encounter 2, starting with max five health would've probably been over turn 1 had Aurim not lucked out and drawn that Health Potion.