You start with your normal hit points and saves etc. Write those down. Do everything pretty much the way you would in your system.
However, when you level up: For each new spell slot you get, roll on this table instead of just filling the slot…
1-59 Gain a spell slot as usual.
60-64 You gain 1 to-hit on all attacks
65-67 All your saving throws improve by 1
68-69 You become more attuned to the secrets of nature. Instead of an empty slot you fill by memorization, you gain a random druid spell of the same level as a daily power.
70 You learn the ancient elven art of spell-cobblery. Taking one day and 100gp times spell level in materials, you can create a pair of shoes that will hold a spell you can cast, until used by the wearer or seven days have elapsed. A second roll of this ability allows 3 uses of the spell before the shoes lose their magic. A third removes the one week time limit.
71-72 You know how you are immune to ghoul paralysis? That’s your inner light protecting you. That light grows stronger. Now wight level drain won’t affect you either. If rolled again follow this progression: mummy rot, vampire level drain, all special damage effects of all corporeal undead.
73-75 Spell dilettante: Instead of gaining the spell slot normally indicated, you gain 2 more slots of the next lower level. If you were rolling for a first level slot, in addition to 2 zero level slots you also gain 2d6 cantrips of your choice to add to your spellbook. (See the original Unearthed Arcana or some other source of 0-level spells.)
76-77 You get good with a traditional elfy weapon. Gain 2 damage with any one weapon.
78 You learned the traditional art of elf baking from your granny. Taking one day and 1,000gp in materials, you may bake a batch of a dozen cookies, all of which have the properties of a single standard potion type of your choice (i.e. something in the first edition DMG or whatever). You can only do this once for each time you roll this entry, and it must be a different recipe (potion type) each time.
79-80 Your horns grow. What, you didn’t know very old and/or powerful elves have horns? Usually they take the form of spirally ram’s horns or deer antlers. Your horns grow d6 inches in length (starting at zero unless you’ve rolled this entry before). For every 4 inches of horn or fraction thereof, you are +1 charisma when dealing with other elves, fairies of all sorts, most forest creatures, and many goblinoid types (but NOT hobgoblins, those guys fucking suck). However, you are -1 CHA when dealing with dwarves, halflings, and any human not of druidic faith. Maximum horn length is 12 inches plus your level. They are not used for combat, you cretin.
81-82 The arcane arts of the elves encompass spells beyond the reach of mere mortals. Add to your spellbook a spell from a supplement or edition not normally used at your table. Its level can be no higher than the highest you can cast. However, you only have 24 hours of real time to research and select your new spell. If you do not email your DM with the complete text of the new spell before the time elapses, you lose this opportunity.
83 You remember a bit of the ancient elven art of making living toys. Pick any monster in the core book that is both small size and possessing no more than half your hit dice. You now have a wooden version of the same as a henchman.
84-85 You grow d12 inches taller. At each whole foot of height above 5’, you gain +1 Str but lose -1 Con as you become a spindly mofo. At Con 2 you must retire from the World of Men, cross the sea, and enter a special home for elvish invalids. If you reach 8’ you can use ogre sized weapons. At 10’ you can start using smaller giant weapons. If you have not previously determined your height, assume you start at 5’6”.
86 You gain a mount or pet.
1 Cooshee (elven dog)
2 Giant Owl
3 Giant Lynx
If you roll this again, you can only get a second of the same type of pet if the first one is still alive. Otherwise, the stink of death is upon you and that kind of monster will never associate with you willingly. The second of the same type with be the other half of a breeding pair and subsequent rolls of the same type will be young (half hit dice, half damage).
87-88 You shrink d12 inches. For each whole foot below 6’, you gain +1 Dex but lose -1 Str. At Str 2 you must retire from the World of Men, cross the sea, and enter a special home for elvish invalids. If you reach 3’6” halfling weapon restrictions apply. At 2’ you can only use daggers and pixie, sprite and certain kobold weapons. At 0 inches tall you shrink out of this plane of existence altogether. If you have not previously determined your height, assume you start at 5’6”.
89-91 The elements possess less dominion over your material body. You now take half damage from a random damage type:
4 Esoteric magical energies such as magic missiles and prismatic walls
However, you also gain a vulnerability, cold iron and blessed/holy weapons do double damage to you. Reroll if this entry comes up a second time.
92-93 Your ears grow improbably long, even for an elf. Gain +1 listen at doors, etc. If rolled a second time, +1 more to listen and halve surprise chances but disguising your elven nature becomes nigh impossible. Reroll all subsequent rolls of this entry.
94 Fear rarely touches elves the same way it does other races. Gain +4 saves versus all fear effects. If you roll this again, you become completely immune to fear, but you also become so inured to death that you lose the ability to ever again acquire new hit points via level advancement. Reroll the third and subsequent rolls of this entry.
95-96 Roll on the crowd-sourced Witch Table curated by Zak
97-98 Roll on Zak’s Ranger Table
99 Once per session when you spend at least 6 hours doing that creepy meditation thing that elves do instead of sleep, you can query the elf hivemind/social media/akashic record. The DM must answer one question, provided that any elf in the past or present would have access to the answer BUT that answer is filtered through the perceptions of the individual knower(s) (i.e. it may not be 100% completely and objectively correct). If rolled again, the number of questions increases by 1, but the number of times per session does not. I.e. a second roll allows a follow-up question in the same meditative trip on one subject.
100 You’ve become so attuned to the elven hive memory that you no longer need to use a spellbook. However, you may only select one spell of each level from among the spells you know, the rest are generated randomly from the complete official elf/MU spell list for the edition you are using. If rolled again two spells of each level may be selected from the spells you know, etc.