- Demilich: first time a non-evil creature enters the demilich's lair they take 3d10 necrotic damage (MM 49)
So, firstly, this only applies if the party happens to have an Evil member, something that most tables don't allow in the first place. Secondly, this involves basically auto damage for approaching an enemy the party might have no idea is there, so many DM's will ignore this, since just randomly causing the players to take damage with no save is sure to trigger reactions and slow down gameplay as they retreat and try and figure out which effect it was that just causes them damage and how they can prevent it.
- Lemure: returns to life in 1d10 days unless killed by a good creature under effects of bless or sprinkled with holy water (MM 76)
There are nearly endless Lemures in the Hells, even if you "kill" one there are millions and millions more of them. This is effectively meaningless hoops. Especially since you could accomplish the same goal by sprinkling the Lemure with Holy Water, no alignment needed.
- Night Hag: evil creatures killed by Nightmare Haunting have soul trapped in bag (MM 178)
Doesn't affect the party at all, because if they are killed by the Hag, the party will be attempting to revive them, and unless they are Evil (which they aren't because most tables don't allow Evil PCs) then they are immune anyways. This even makes them less useful as a plot device, because any soul the Night Hag has is evil, and therefore not someone the party will really care about saving.
- Lycanthrope: PC becoming lycanthrope has alignment changed to appropriate alignment (MM 207)
Pointlessly forcing a character to be something they weren't by dint of a curse that most players want removed as soon as possible, because it isn't fun to be forced to attack and kill their party members or be forced to be an NPC.
- Rakshasa: vulnerability to piercing damage from magic weapons wielded by good creatures (MM 257)
Pointless hoop jumping. Especially since it just makes them weaker and requires defining if you are "good enough" to have them take double damage. Which, if the players know about, will just have them extolling all their virtues. Nothing is lost by ignoring this.
- Shadow: non-evil humanoids killed by Strength Drain become shadows in 1d4 days (MM 269)
Again, does nothing if no one in the party is Evil. And, even if they are Evil, they will likely have been hiding that fact, leading to them being treated the same way. Seems pointless
- Sprite: can detect creature's alignment on failed DC 10 cha save (MM 283)
Detecting Alignment doesn't do anything if alignment doesn't mean anything. Also, as someone with a Chainlock Sprite Familiar, I'll go ahead and let you know that A) Getting this to land is nearly impossible without tipping your hand, because the Sprite becomes visible and the target knows what is happening and B) 80% of the time, it is pointless, because I can tell who my enemy is.
- Unicorn: regional affects maximize healing and suppress curses on good creatures (MM 293)
Never really encountered a Unicorn, but these seem like really mild effects. And raises an amusing point. If you are cursed to be Evil, is your curse suppressed? Like say, your alignment changed to Evil because you became a werewolf, now you are Evil and can't have the curse that made you evil suppressed. It could be amusing though, so we'll give a +1 for actually finding something that matters
- Vampire: PC becoming vampire has alignment changed to lawful evil (MM 295)
They also become a thrall unable to reject the commands of the master vampire, can't cross running water, and burn and die in the sun. None of this is worth the trouble and if the dead player wasn't made an NPC, they are going to seek to remove this curse as soon as possible, when they are allowed to do so by their utter mind-control by the master vampire. Which, by the way, can only be broken by drinking the master vampire's blood.
- Gold Dragon: regional mist assumes ominous shapes to warn non-evil creatures of evil creatures present (PHB 115)
All this amounts to is a way for the DM to signal to the players "bad guys approaching" which... if they want to do, they will do anyways. Pointless.
- Candle of Invocation: creatures with same alignment as candle's deity have advantage on attack rolls, saving throws and ability checks, clerics or druids with same alignment can cast 1st level spells at-will (DMG 157)
A single item that is either very powerful or utterly useless. Might as well just have it act on allies of the Diety, since the only time this is going to be more than "one side of the fight is super buffed" is when good people are fighting good people, and someone activates the candle. And you should have given plenty of other signs that that was happening. Also, "here is a powerful magical item, you can't use it because you need to be Neutral Good to use it and you are Lawful Good" is kind of just a jerk move.
- Book of Vile Darkness: non-evil make DC17 cha save or alignment change to neutral evil (DMG 222)
"Hey, I just removed your PCs agency by forcing you to be something you aren't". Always a terrible idea.
- Book of Exalted Deeds: requires attunement by a good creature, evil creatures take 24d6 unavoidable radiant damage (DMG 222)
If no one is Evil, then this does nothing. And anyone powerful enough to be going after this artifact isn't stupid enough to attune to it and nearly die in the process. So, basically plot armor for why the bad guy can't use the good relic.
- Blackrazor: requires attunement by a non-lawful creature (DMG 216)
Seems to be another pointless resctriction on who can use the cool magical item
- Deck of Many Things: Balance card reverses alignment (DMG 162)
Taking away player Agency by forcing them to be something they aren't again. Also, changing alignment doesn't do anything except completely rewrite a player character, so no loss in no longer being able to force people to become NPCs
- Talisman of Pure Good: requires attunement by a good creature, neutral/unaligned creatures take 6d6 radiant damage, evil creatures take 8d6 radiant damage (DMG 207)
Pointless resctrition on who can use the cool magic item. Also, this item has only a single power that can be used if you aren't using it as a spellcasting focus. "Kill evil creature". So... Kind of a pointless item anyways, unless you want to have your final boss one-shotted.
- Talisman of Ultimate Evil: requires attunement by an evil creature, neutral/unaligned creatures take 6d6 necrotic damage, good creatures take 8d6 necrotic damage (DMG 207)
The bad guy version, great for giving an item to the villain that can one-shot PCs, preventing them from being resurrected (no remains) and then denying the party loot. Why you would even want this item in the game, I have no idea. Seems like terrible DMing, unless the party is high enough level to circumvent the item via True Resurrection or Wish
- Robe of the Archmage: requires attunement by a character of the appropriate alignment (DMG 194)
Only useful if you want to deny the party loot, but still want the bad guys to have magical items.
- Moonblade: requires attunement by a neutral good elf or half elf (DMG 217)
Pointlessly restrictive way to either make sure only a single party member can use the loot, or to prevent anyone from using it.
Classes
"Here are classes you can deny access to, in the DMG, because only 'bad people' have these abilities." . They are also in the DMG. It was literally just a suit of options for making villains, and the alignment restrictions are kind of pointless.
- Mount Celestia: good creatures gain effects of bless while on plane and lesser restoration on long rest, optional (DMG 59)
Buffing the party if they go to Mount Celestia, where I have to wonder who they are fighting? Like, Bless only works if you are attacking or saving, and if you want everyone blessed for a fight, there are other ways to do it, plus, it only blesses some of the party if people are neutral, and that feels a bit weird if they are, say, fighting to protect the place.
- Bytopia: DC 10 wis save or non-lawful/neutral good creatures changed to one of these alignments, optional (DMG 59-60)
"Here, I altered your character for you, now you have to play this character"
- The Abyss: DC 10 cha save or become corrupted, corrupted creatures become chaotic evil after 1d4+2 days, optional (DMG 62)
"Here, I altered your character for you, now you have to play this character"
- The Nine Hells: DC 10 cha save or become lawful evil, optional (DMG 64)
"Here, I altered your character for you, now you have to play this character"
- Mechanus: DC 10 wis save or become lawful neutral, optional (DMG 66)
"Here, I altered your character for you, now you have to play this character"
- Spirit Guardians: good and neutral casters do radiant damage, evil do necrotic (PHB 278)
Since "No Evil PCs" is the rule, this is pointless. And, I can't think of any point to making evil characters deal necrotic damage except to weaken the spell, since it makes it easier to get resistance.
- Glyph of Warding: can trigger based on alignment (PHB 245)
It can also trigger off proximity. The only point in having it be based off alignment is so you can have an explosive trap the PCs don't trigger, which is pointless because....
- Nystul's Magic Aura: masks alignment to magical effects that sense alignment (PHB 263)
A 2nd level spell removes every single item restriction and makes Glyph of Warding based off alignment pointless
- Ceremony: Atonement restores alignment of creature that has had alignment change (XGTE 151)
And a 1st level spell to reverse all the forced alignment changes that you shouldn't have been doing anyways.