Tomb of the Serpent Kings by Skerples, with art by Scrap Princess, is an awesome dungeon adventure which you should immediately grab if you haven't already.
It's a very good dungeon per se, and is also famous for being a "learning" module both for players and GMs who want to familiarize with the OSR style of play.
It is available as a free pdf, but I've grabbed the A4 Print-on-Demand version and I'm very happy with it. I'm preparing to run it for a group of total newbies, and see how good it is at its intended job!
And since I'm going to use Old-School Essentials, I've prepared a few adaptation notes, so why not share them?
Also, check this cool, free isometric map of the tomb, by Andrew Duvall!
Here are the notes for the Lower Tomb.
Tomb of the Serpent Kings - Lower Tomb
Notes for Old-School Essentials
Treasure: All values should be multiplied x10!
Wandering Monsters table:
Fungus goblins are just OSE goblins (but sticky!)
Large spider: it says it's fist-sized, so it's not really any of the giant spiders listed in OSE.
Armour Class 7 
Hit Dice 1/2 (2hp)
Attacks 1 × bite (1d3, +1d4 poison if failed save)
THAC0 19 [+0]
Movement 120’ (40’)
Saving Throws D12 W13 P14 B15 S16
Room 22: Not an adaptation note but I believe, based on the textual description, that the map is wrong about the position of the door, as it seems to me that the door is to be interpreted as "Recessed 5' into the wall" from the side of the chasm, not from the room 23 side. Anyway, since the stone hammer doesn't crush the victim against a solid object, I believe the trap should be save vs death (+2) or 1d6 damage, plus another save vs death or fall into the chasm (instant death).
Room 24: The infamous skeleton jelly: as per skeleton in Old-School Essentials, except immune to all damage, and only inflicts 1d4 damage. I've decided it can be turned by clerics even though it is against the idea of the encounter, because I don't feel like being a jelly makes it more powerful than the gods, and when in doubt I always choose in favor of the players.
Room 25: Save vs death or d6 from falling, and again for the spikes at the bottom.
Room 26: Easy to pick: double % for thieves to pick the lock. Also, rusted, so I guess it can be kicked open like a stuck door.
Room 30: For characters inside the pit, instead of COS damage I'll have a simple save vs paralysis to avoid becoming unconscious (and falling into the flames for 2d6 damage).
Room 32: Baltoplat the succubus!
I've mixed the statistics of a harpy from OSE with those of succubi from AD&D 1st edition. I suppose a succubus should have more special abilities, but I think this is enough for this specific encounter.
Baltoplat the Succubus
Armour Class 2 
Hit Dice 6* (27hp)
Attacks 2 × claws (1d3)
THAC0 14 [+5]
Movement 60’ (20’) / 150’ (50’) flying
Saving Throws D10 W11 P12 B13 S14
Morale 10 (but will flee combat)
Immunity: fire (normal & magic) and non magic weapons.
Energy drain: A kiss from Baltoplat drains one level (save vs death to avoid), like wights and similar.
Spells: Charme, minor illusion.
Room 33: The gas trap is save vs death or d6 damage.
Room 34: I guess the eggs can be sold to sages etc for 1d6 x 10gp each. Not very useful for warmblooded creatures after all, but still a curiosity item. Or, they may become more useful for expeditions into very cold environments if the heating effect is increased to become equivalent to that of a decent-sized bonfire (but without the flame).
Room 35: Save vs death for the blade traps.
Room 38: See basilisk in OSE. Definitely going to use the 2-round petrification routine as suggested in the module (instead of the OSE save-or-die RAW petrification rules), as it makes for a more engaging and less brutal encounter.
Room 42: Xiximanter the lich! Liches can be found in the OSE Advanced Fantasy monster book, but I want to make him a bit different (and sligthly less powerful). Spells: I've used the online OSE random tools to determine the spells he has prepared, but also figured a shorter list for those who use Wondrous Weavings Warped & Weird (this second list is handpicked).
Xiximanter the lich
Armour Class 1 
Hit Dice 10**** (45hp)
Attacks 1 × claw (1d8 + paralysis)
THAC0 11 [+8]
Movement 60’ (20’)
Saving Throws D6 W7 P8 B8 S10
Undead: Make no noise, until they attack. Immune to effects that affect living creatures (e.g. poison). Immune to mind-affecting or mind-reading spells (e.g. charm, hold, sleep).
Immunity: Non-magic weapons, electricity, cold, all spells that cause metamorphosis, madness, death.
Fear Aura: Xiximanter may decide to "turn on/off" his fear aura. Those who see it must save vs spell or flee for 2d6 turns. Characters level 4 or higher are immune.
Paralyzing claw: Save vs. paralysis or paralyzed for 6 turns.
Level 1: Floating Disc, Hold Portal, Sleep, Floating Disc
Level 2: Locate Object, Detect Invisible, Wizard Lock, Mirror Image
Level 3: Lightning Bolt, Protection from Evil 10’ Radius, Invisibility 10’ Radius, Protection from Evil 10’ Radius
Level 4: Remove Curse (Curse), Wall of Ice, Polymorph Others, Hallucinatory Terrain
Level 5: Animate Dead, Hold Monster, Animate Dead
Level 6: Reincarnation, Part Water, Part Water
Alternate Spells (if using Wondrous Weavings Warped & Weird): Flesh to stone, anti-magic shell, death spell, disintegrate, teleport, hold monster, wizard eye, lightning bolt, invisibility, fly, magic missile, sleep, web, mirror image
Room 45: Random potions! Here's what I got from the online OSE random tools, plus the ones listed in the module.
I've decided to ignore the "spell mutation" potion listed in TotSK, because I cannot figure out what it is and this is a rich hoard of magic anyway. I made the list into a d20 table just in case the players pick a random bottle.
d20 color potion type
1 white diminution
2 grey flying
3 grey flying
4 black control undead
5 transparent levitation
6 transparent levitation
7 orange heroism
8 orange heroism
9 red control animal
10 brown giant strength
11 green speed
12 yellow fire resistance
13 yellow fire resistance
14 white diminution
15 purple healing
16 purple healing
17 purple healing
18 pink poison
19 gold longevity
20 gold longevity
Room 46: The cursed throne. I'll have it be worth 8,000gp. Players must either solve a puzzle or cheat (or kill!) Xiximanter, AND decide to quit the expedition to take it away, and go back to town at a movement rate of probably 6 miles/day, meaning more chances for random encounters. This deserves to be a big, memorable jackpot.
The curse: save vs spell, or "desire lordship and conquest", as stated in the module. How to translate this into game terms? I'm frankly considering just going with a (possibly false) hallucination/vision of the future as a ruler, and let the player fall for it or not. Something like
"you see yourself sitting on this very throne, but you're visibly older, with wrinkles and grey hair, and you're dressed in rich, lordly clothes and have a jeweled crown on your head. A young valet bows at your feet and says the tributes from the eastern provinces of the kingdom have arrived and include an enchanted ring as a gift from your vassals."
Room 48: Save vs poison or flee with disgust.
Room 50: Love this room.
Dredging the soil requires 1/turn per 3x3' square per person, with a roll on this table to see what comes up. Each result can only come up once, and I consider only 6 squares to be the goblins' garden, so this all means digging it all up requires 1 hour if only 1 person is doing it, or less if more characters work at it.
1 A finger and a fork
2 A rusted dagger
3 1d20 gold pieces
4 A ruby worth 300gp
5 A chicken bones
6 The crown!
The crown: If worn, save vs spell etc. The crown is the other "jackpot treasure" in the module, and players need to figure the soil in the room might hide something precious. I'll make it worth 5,000gp.
Appendix 1: Where is Xiximanter's spell book?
I guess Xixi would keep it in either room 44 or 45, perhaps inside a secret compartment in the wall, hidden as a beautiful fresco of snake men empowered by lightning.
Appendix 2: Where is Xiximanter's phylactery??
That's a very good question, one that expert players may have. Truth is, OSE liches haven't got it!
So it's up to you if you want to add one or not. I would hide it in a secret compartment inside one the pits in his rooms.
Appendix 3: What if... Xiximanter is a divine caster??
It would be a cool alternate version! Here's a selection of memorized spells from cleric's lists, if you want Xixi to be a divine type lich.
Level 1: Protection from Evil, Light (Darkness), Purify Food and Water, Detect Magic, Purify Food and Water
Level 2: Hold Person, Know Alignment, Resist Fire, Speak with Animals, Silence 15’ Radius
Level 3: Locate Object, Cure Disease (Cause Disease), Cure Disease (Cause Disease), Cure Disease (Cause Disease), Remove Curse (Curse)
Level 4: Speak with Plants, Sticks to Snakes, Neutralize Poison, Speak with Plants
Level 5: Raise Dead (Finger of Death), Commune, Commune, Quest (Remove Quest)