Also, I’ve tinkered with trying to do WHF in 5E. Here’s what I would recommend:
No magic above 6th level spells.
Alternatively, allow level 7-9 spells, but all spell failures are crit fails.
No spell slots, spell preparation, etc. All spells are cast at will (assuming you are high enough level to cast and know that spell.)
Spells can be scaled (ie, a level 1 spell cast as a higher level spell) as usual. Treat the spell as though it were the level cast for purposes of spell failure/crit failure.
Add spell failure mechanics: Roll a d20. If the number is less than or equal to the spell’s level, the spell fails. If it’s a nat 1, it’s a critical fail. Resolve the spell as normal, including crit failure if it has an attack roll.
Cantrips and Level 1 spells can only crit fail.
For abilities that rely on spell slots, instead use Spell Points (DMG 288-289). Divide the recommended number of spell points by 4 (minimum 2). Not that abilities that burn spell slots are not spells and thus do not risk failure/crit failure.
For Critical Hits, use max damage dice + roll all damage dice again.
Create Crit fail tables to roll on.
Use Crit fails for all attacks rolls (nat 1). These should be inconvenient.
Use Crit fails for all spells (see the Spells section above). These should be dangerous. Add the spell’s level to the dice roll when determining the Crit fail effect for a spell (assuming 20 is the worst effect on the table, and 1 is the most manageable.) Note that Cantrips add 0 to the roll.
Use fear and horror rules (DMG 266) as appropriate (especially for Chaos)
Use sanity rules (DMG 265) as appropriate (especially for Chaos)
Use massive damage/system shock (DMG 273)
Use lingering injuries rules (DMG 272)
Players aren’t incapacitated at 0hp. Instead, players must make a Death Save for every -(2*CON Mod) HP. PCs who fail a death save must roll on the lingering damage table (DMG). PCs die after 3 failed Death Saves. Death Saves reset after a long rest. Example: a PC has 18 CON (+4), and 2 HP. He takes 12 damage. This drops him to -10 HP, which is over the -8 threshold. He makes a death save, which he passes. The next round, he fights as normal, but takes an additional 20 damage, reducing him to -30hp. He makes two additional death saves (one for crossing -16, and one for crossing -24), which he fails. He rolls twice on the lingering damage table in the DMG.
Treat bosses/solos/elites as PCs, but let normal monsters die at 0hp.
Use the Healer’s Kit Dependency and the Slow Natural Healing rules (DMG 266-67)
Use the Gritty Realism rules for rest (DMG 267)
Allow PCs to make a melee Opportunity Attack if any enemy within 5 feet of them makes an Attack. Do not give this to monsters or PCs with ranged weapons. This is should make melee higher on the risk/reward spectrum. (There are some class features that trigger on similar conditions. Tell your players to skip them.)
Treat anyone in melee as being in half-cover.
Make sure there is plenty of cover available, and expect lots of ranged fights.
Ranged weapons (bow, crossbow, etc) do not add Dex mod to damage (STR-based thrown weapons do)
There are none before Level 11.
Only Very Rare and Legendary magic items exist. And I guess maybe a few potions or the occasional scroll.
Everyone gets the Lucky feat for free. Luck points recharge at the beginning of the session. If a PC dies, he can “burn” luck. He is no longer dead, but is instead incapacitated but stable, suffering all the conditions he had when he died (lingering injuries, system shock, etc.) Reduce the number of Luck points he receives going forward by 1. If a player has a maximum luck points of zero, he cannot burn luck.
There are no other Feats.
There are no multiclass restrictions
Gain all armor/weapon/tool proficiencies of all classes (NOT skills)
Extra Attacks stack. If you are Barbarian 5/Paladin 5/Fighter 5/Ranger 5, you make 5 attacks (1+4 extra). If you are Barbarian 5/Fighter 11, you make 4 (1+ 3 extra). Barbarian 4/Fighter 1 still makes only 1 attack.
Combat will be deadly, even at high levels. The party should never get into combat without a plan. Bad dice will easily wipe the party, which is why luck points exist.
Casters are extremely dangerous. A PC who only casts spells cannot survive; all PCs will need a conventional weapon, and should be saving spells for major threats, especially when they are out of luck points for the session.