Here. Example of how the Intellect Devourer battle should go on the beach with proper D&D 5e rules and stats, 4 Players + Shadowheart for a party of 5. All at level 1 still.
Initiative = Shadowheart rolled 20 total, then Lynari 18, Intellect Devourer 2 (ID2) 17, Diadell 16, Vel 13, Drogyn 3, ID1 3, and ID3 3.
Shadowheart keeps her distance and rolls a 20 with her Guiding Bolt. Hit. 15 Damage off of ID1. It had 22 HP total to begin with. It now has 7 HP remaining.
Lynari is a drow wizard and casts Magic Missile at ID1. Rolls 3d4, does 6 damage. ID1 has 1 HP left.
ID2 uses Dash and runs 80 feet, moving up to Drogyn who is towards the front.
Diadell, a wood elf Rogue, moves up and attacks ID2 with her two shortswords. Sneak Attack proper rules = Advantage is not necessary if an ally is within 5 feet of the target. Since Drogyn is there, she gets to use Sneak Attack if she hits. First roll 1. Critical Miss. Second roll 14+5=19. Hit. 1d6 damage = 4, but ID2 has Resistance to Piercing and Slashing weapons. Half damage. Diadell only does 2 points off. ID2 had 25 HP. It now has 23.
Vel is a half elf Druid. He casts Entangle and effects an area that captures both ID1 and ID3 in it. Both must make Strength checks against Vel's Spell DC or be ensnared/restrained for up to 10 rounds. Each round, a successful check could free them. Vel's DC is only 12. ID1 rolls a 1. ID3 rolls a 4. They both fail and are restrained.
Drogyn is a half-orc barbarian. He attacks ID2 with his greatsword and rolls a 4+5=9. Miss. He needed a 12.
ID1 rolls to break free of the Entangle spell. Rolls a Natural 20 and succeeds. That cost an Action. It can now only move 40, but because of difficult terrain (the Entangle spell area), it cannot move fast enough to get up to the adventurers.
ID3 rolls to break free of the Entangle spell. Rolls a 10-2=8. (Strength is only 6, which is a -2 bonus). It fails and can't move.
Round 2. Shadowheart casts Guiding Bolt again on ID2. She rolls a 9+3=12. Hit. She rolls 4d6 and does 10 damage. It still has 13 HP.
Lynari casts Magic Missile again. She hurls 1 at ID1 and the other 2 at ID2. She does 1 point of damage to ID1, killing it. She does 4 to ID2. It has 9 left.
ID2 attacks Drogyn. Here is how an Intellect Devourer should behave. First, attacks with claws. Rolls a 2+4=6. Miss. Has Multiattack, so it can attack with claws and Devour Intellect in the same turn. Uses Devour Intellect on Drogyn. He must make a DC 12 Intelligence saving throw or take 2d10 psychic damage. He rolls a 9+0=9. Failure. DM rolls 2d10 and only gets 3. He's lucky. He had 15. He now has 12 HP. But that's not all. The DM also rolls 3d6. If the total equals or exceeds Drogyn's Intelligence score of 10, that score is reduced to 0. The target is stunned until it regains at least 1 point of Intelligence. DM rolls a 12. Drogyn is out of the fight. His Intelligence is now 0.
Diadell's turn. She attacks twice, once with each shortsword. She rolls a 2 and 4. Miss both times. Vel runs up and attacks ID2 with his spear. He rolls 6+3=9. Miss.
ID3 rolls a 14-2=12 and finally breaks free of the Entangle spell. It moves through difficult terrain, unable to reach the heroes.
Round 3. Shadowheart has no spell slots left. She pulls out her crossbow as a free action and fires at ID2. 11+3=14. Hit. 1d8+1 damage, she rolls a 7+1=8, but ID2 has Resistance. Half damage. She only actually deals 4 HP off. It has 5 HP remaining.
Lynari casts Firebolt. Natural 20. 2d10 damage (using extra d10 because of Critical Hit). She rolls a 9 total. ID2 dies in a fiery ball of death.
Diadell sheaths her swords and falls back 30 feet. She pulls out her bow and fires. This is all 1 Free Action. She hits with a Natural 20. 2d6 damage. 6+3=9. No Sneak Attack because no advantage. Resistance means half damage. ID3 loses 5 HP. It had 20. It now has 15.
Vel has no attack to use that won't bring the creature closer to him and the others, so he falls back.
ID3 uses Dash to move 80 feet, but is unable to get close to the remaining heroes because they moved further away.
Round 4. Shadowheart fires her crossbow again. 11+3=14. Hit. 3+1=4 divided by 2 (Resistance) = 2. It has 13 remaining.
Lynari casts Firebolt again. She only rolls a 4 to hit. Miss.
Diadell rolls for her bow and misses with a 2.
Each of them moved 30 feet further away to escape.
Vel risks his life to keep ID3 away from the others. He runs 30 feet and uses Thorn Whip. He rolls a 1. Critical Miss.
ID3 takes the bait and charges him. It attacks with claws. 4+4=8. Miss. It uses Devour Intellect. Vel rolls an 18+1=19 to resist and succeeds. He only needed a 12. He takes no damage.
Round 5. Shadowheart fires again with 11+3=14. She does 6+1=7 damage divided by 2 = 4. It has 9 remaining. She moves another 30 feet away to hopefully keep it at a distance.
Lynari casts Firebolt. 16+4=20. Hit. 8 damage. ID3 only has 1 HP remaining.
Diadell fires her bow. Natural 20. 2d6. She rolls a 6. Resistance reduces it to 3. That is enough to kill it.
Party gains 270 XP each. From the Imp fights during the Prologue, they received 600 XP during the first fight divided by 5 (including Lae'zel) = 120 XP each. After meeting Shadowheart, they had a party of 6. They faced another group of 2 Imps and a Hellsboar on the bridge. Each imp was 200 XP and Hellsboar was 100. So that was 300 XP total. Then they faced a second group of the same for another 300 XP total. Divided by 6, that was another 100 XP each. So, by the end of the Prologue, they had 120+100=220 XP. The DM then awarded them 50 XP for completing the Prologue for a total of 270 XP. They needed 300 XP to level up.
So, they were able to level up after the Intellect Devourer fight, having a total of 540 XP.
Notice how this fight was not so tough with a party of 4 + Shadowheart. They still lost a man who would now only recover during a Long Rest; thus the Long Rest would be suggested by the game at that point because you have a fallen companion who wouldn't recover until after the Long Rest from Devour Intellect. Thus, it makes sense to do a Long Rest Tutorial at that point especially if that battle didn't go as well as this one did.
First, what part of this fight here couldn't be done in a video game? Anything?
Second, THAT is how Intellect Devourers should act and how tough they really are.
Third, tell me that the game wasn't designed with a party size of 5+ based on this scenario above. The ONLY reason it works for a Party Size 4 at all (and ESPECIALLY for just 1 MC and Shadowheart) is because they not only reduced each of the ID's to like 10-12 HP, but they removed Resistance and Multiattack and took away their Devour Intellect and Body Thief abilities.
This is why I'm suggesting:
1. Option for Party Size 6. This Option allows 4 Player Multiplayer games to have their 4 Custom Characters and Shadowheart, thus giving them a better chance of surviving this fight. 2. Option for Core D&D 5e Rules Difficulty so that when you play 4 Player Multiplayer + Shadowheart, it is a challenging and rewarding experience instead of an absolute breeze that is lame and almost pointless. It also gives the Intellect Devourers their proper stats and signature attacks so that they are actual Intellect Devourers. 3. Option for Players to be able to create up to 4 Custom Characters even in Single Player Mode so that they could play with the Core D&D 5e Rules Difficulty from the beginning of the game and not be totally wiped out by fighting 3 Intellect Devourers with only 1 MC and Shadowheart.