Tenser's TSR Transformation

This is the exact description as provided in the earliest edition called "Advanced Dungeons & Dragons" as sold by TSR. 

Only the tray at the top had been refomatted for readability.  Some color has been added for readability.
Markers have been inserted for reference to the converted results (alphabetic for the spell header, numeric for the main text). 

Only use this in your compaign if players are willing to enjoy the powers and endure the risks of slower castings with grander power - that can be interrupted mid cast.

Magic-User spell

Tenser's Transformation (Alteration-Evocation)
Level: 6th 🄰
Range: 0
Duration 1 round / level 🄱
Area of Effect: Personal
Components: V, S, M 🄲
Casting Time: 6 segments 🄳
Saving Throw: none

Explanation/Description: Tenser's Transformation is a sight guaranteed to astound any creature not aware of its power ①, for when the magic-user casts the dweomer, he or she undergoes a startling transformation.  The size ② and strength ③ of the magic-user increase to heroic proportions, so he becomes a formidable fighting machine, for the spell causes him to become a berserk fighter ④! The magic-user's hitpoints double ⑤, and all damage he sustains come first from the magical hitpoints gained ; so if damage does not exceed original hit points, none is actually taken ⑥, but if damage beyond the additional amount is sustained, each point counts as 2 (double damage) ⑦. The armor class of the magic-user is a full 4 factors better ⑧ than that he or she possessed prior to casting the spell (AC 10 goes to 6, AC 9 to 5, AC 8 to 4, etc.), all attacks are at a level equal to those of a fighter ⑨ of the same level  as the magic-user (i.e. the spell caster uses the combat table normally restricted to fighters), and although he or she can employ a dagger only in attacking ⑩, damage inflicted by the weapon is at +2 additional hit points ⑩, and 2 such attacks per round ⑪ are made by the magic-user. However, it is worth noting that this spell must run its full course 🄱, and the magic-user will continue attacking until all opponents are slain, he or she is killed, the magic is dispelled, or the Transformation  duration expires.  The material component for casting this dweomer is a potion of heroism (or superheroism), which the magic-user must consume during the course of uttering the spell.

 What follows is a conversion for those exact features highlighted.

A. TO CAST THIS SPELL USE EITHER OF THESE METHODS

• use a modern level 6 spell slot, and then wait 6 rounds for effect.
• see 🄳

B. IN THOSE DAYS "ROUNDS" WERE A FULL MINUTE LONG

• the caster's transmutation into a supersized capable Barbarian or Fighter will last as many Minutes as their level.

C. COMPONENTS - VERBAL, SOMATIC, MATERIAL.

Spells can be interrupted if the conditions for their casting aren't good throughout the gathering of the arcane energies.

• If in the Reaction burning or waiting phase, you become Silenced or otherwise unable to work verbal magic.
• If during the same period you become incapable of making even the simplest gestures (Paralyzed or Unconscious for instance),.
• If somehow during 6 entire rounds, you cannot manage to quaff the Heroism or Superheroism potion you need for to prime the magic.

No powers normal to the potion are in effect; the spell consumes it, not the drinker.
Also, one can't merely use the hair of a bull, or what happens is the more modern version of the spell instead;  time or personal energy that got poured in would be wasted.

• The spell slot of any incomplete or interrupted spell is still lost.

D. AND "SEGMENTS" WERE SIX SEECONDS LONG, WHICH WE CALL A ROUND NOWADAYS.

• It may be cast without spell slot by a caster capable of 6th level spell use,:if at the beginning of their turn, for each of 6 rounds in sequence, they burn their Reaction.
• Reactions didn't exist in the original AD&D and the general reactive give and take of combat within a minute was managed differently then..
• However spell casting could be interrupted, without benefit of any saving throw on the magic user's part, (just like in the movies) if the criteria of their spell were ruined before the cast was complete, so that's described above.

E. FOR EXTRA.  AND FOR EXHAUSTION.  AND FOR HOW TIRED YOU MIGHT BE BY THE TIME YOU FINISH READING THIS SPELL.

During the waiting period or the 6 rounds of reaction burning the caster may spend a level of Exhaustion to force the magic to take effect iIMMEDIATELY, as long as the correct potion has already been ingested. There was no such thing as Exhaustion in the original AD&D (the closest being a recovery period after being raised from the dead), but it's the normal resource Barbarians have to burn with Raging, so it's thematic.  If this is done, the Exhaustion doesn't happen until the spell wears off (just like the modern version, except you made your choice, so you can't resist it).  If Reactions are used to fuel the spell, they still cannot React until all 6 rounds have gone without, but all other effects apply fully.

1. Each foe you battle acquires Disadvantage to their Initiative roll
   (This is the normal 5e impact of the Surprise condition, although it normally only happens before combat begins.)
   Disadvantage does not stack so this impairment only occurs once per foe.
   Effect happens after you melee them and can be considered tactical maneuvering as if imposed by a warrior trained in combat for many years.
   
   This feature has no effect on a foe that has survived a combat where a previous Tenser's Transformation caster waged battle.  Not even if it was the other sort.  It was creepy enough the first time and just doesn't surprise them anymore.
   
   
2. You are able to lift or carry things as if one Size larger.
   For a typical character (Medium size humanoid) this means you now control a 10' x 10' space (ref basic rules), unarmed combat does 1d4 greater damage
   (ref. .the Enlarge portion of the Enlarge/Reduce spell, although only you are changing size).
   
   You can wield items comfortably that suit the new size.
   If you are wearing armor, it still aids you, but it may need mending afterwards.  How uncomfortable it is may be hidden by the inherent fury of the spell.
   
   
3. For the active duration of the spell, our strength becomes that of a hero.  Pick only one:
   • Epic Combat Prowess
     A fighter or barbarian related stat (DEX, STR, or CON) goes up by 1, up to a maximum of 30.
     You have Advantage with Martial and Simple Weapons.
     Each round for one attack only, if it misses you can force it to be a hit.
     
     
   • Giant Strength:
     Your STR becomes 23, which makes a STR mod of 6. 
     You can also lift any things a giant can easily lift.
4. Because modern AD&D has distinct classes for Barbarian and Fighter, only one will apply.
   In any case, fighters in the original AD&D did not cast spells and any spells not already cast cannot be cast by the transformed magic user.
   When taking the Attack action they can attack twice (3x at 11th, 4x 20th).  See also point ⑪.
   
   
   • If the affected caster is WEARING ANY ARMOR, gain the class effects of a Champion Fighter.
     The "berserk" aspect will be covered by the continued battle stance and very long spell effects.
     • +1 Critical Range (normally 19-20 instead of 20).  If past 15th level +2 Critical Range (normally 18-20).
     • Add half Proficiency if not already proficient, to any STR, DEX, or CON check.  That includes Initiative, which is a DEX ability check.
     • Great Weapon Fighting: with a versatile (2-handed use) or 2-handed weapon, if a 1 or 2 is rolled on a damage die, it can be rerolled and the second must be kept.
     • Two Weapon Fighting: with two weapons in hand, the related ability mod is also applied to damage with each weapon.
     • Second Wind: 1 time in the fight the caster can use a Bonus action to recover 1d10 + their level in hitpoints.
     • Action Surge: 1 time in the fight (2 times if 17th level or higher) the caster can take an additional Action on their turn.
     • Indomitable: 1 time in the fight (2 times if 13th, 3 times if 17th) they may reroll a failed saving throw.
     • If of 18th level or higher a Survivor regains hitpoints of 5+ CON mod on any round where they have less than half their normal total,
       but more than zero hit points remaining.
   • If the affected caster is NOT WEARING ARMOR, gain the class effects of being an enraged Barbarian:
     • enraged:
       • Resistance to Bludgeoning, Piercing, and Slashing damage
       • When making an attack using STR, gain bonus damage (this would normally be according to a table.  The table is confusing. Also not relying on ⑨ )
         equal to normal PB minus 2.
       • The transformed caster has Advantage on STR ability checks and on STR saving throws.
       • The first skill among these used channels primal rage and therefore plus STR mod to:
         Acrobatics, Intimidation, Perception, Stealth, or Survival.
         This skill retains the bonus throughout the remainder of the spell.
       • The caster is however, so angry and intense that they cannot Concentrate to maintain any ongoing spells either.
       • If Incapacitated  but not slain, the rage will fade to a mere +2 on STR related things, and the Resistance will be lost.
       • if brought to zero hit points but not slain, make a DC 10 CON saving throw. On success hit points instead become 2x of level (not striking zero after all, one is neither Dying nor Unconscious) and the raging battle continues.
         The DC goes up 5 each time this is used until the spell is over. 
     • Regardless of Rage status, the caster also gains
       • Feral Instinct.  Take immediate Advantage to Initiative. 
       • DEX and CON mods as bonus to Armor Class. 
         It might end up somewhat insane, but remember, critical hits and area of effect damage are still feasible ways to get injured.
       • Danger Sense. Such a warrior has an uncanny sense of when things are not as they should be, and thus has Advantage on DEX saving throws.
       • Speed increased by 10 feet.
       • (A wizard is already reckless enough using this instead of their spells, and choices are complicated.
         Reckless Attack and its options regarding Brutal Strike do not apply.)
       • If the caster is level 18 or higher, their STR score can be used as the score if a total for a STR based ability check or saving throw is less.
       • Mastery properties of weapons that the magic-users of old were permitted to wield, the arcane and the fury intermingling.
         Mastery properties were added in the 2024 rules set of 5e AD&D and this can be ignored if your campaign does not use them.
         
         • Nick (Dagger). When an extra attack is with a weapon of the Light property, make yet another attack wihtout having to spend a Bonus action. 
           This can only be done once per turn.
           
         • Vex (Dart). On a hit, if it succeeds at doing damage to a creature, then gain Advantage against them on one's next attack made
           before the end of the caster's next turn.
           
         • Topple (Quarterstaff). On a hit, force a target to make a CON saving throw (DC 8+STR mod+Proficiency). Should the foe fail, they fall Prone.
5. The caster gains Maximum Hitpoints worth in extra hit points.
   Strictly speaking these are true hitpoints (from a bigger, tougher You) and therefore they can be healed just as you can. 
   
   
6. Once the spares are gone they are completely consumed, just as temporary hitpoints normally are.
   
   
7. Should the extras be thusly consumed, take double damage from any further injuries until the spell ends.
   (Vulnerable, to all damage without relation to type. If anything else attempts to make you Vulnerable, it doesn't stack.)
   
   
8. +4 AC which stacks with other benefits.
   
   
9. Bonus to attack rolls made with simple and martial weapons based on class was different; the tables changed at different markers.  Instead of digging up an old table or trying to translate the THAC0 logic, consider the Proficiency Bonus related to weapon wielding to be 1 per 3 levels instead of 1 per 4 levels.  By the time someone's able to enjoy this spell, that's a pretty strong boost.
   
   
   • 1-3:     1
   • 4-6:     2
   • 7-9:     3
   • 10-12: 4
   • 13-15: 5
   • 16-18: 6
   • 19-20: 7
10. Modern spellcasters don't restrict their weaponry in entirely the same way, but anyone in every era agrees that daggers are sharp. When you hit a target with a attack with dagger, that target takes an extra +2 per die roll.
    
    
11. Did we mention daggers are sharp?  They're also fast.  Wielding a dagger gets you an additional strike one time when you use the Attack with it or Reaction for an Opportunity Attack with one.  Yes, this stacks with the Mastery property if it applies from above.  Terrifying?  Good. 
    
    
12. (difference from the modern form) You do not gain any proficiencies with equipment.  If you already had such skills you don't lose them.  The advantages and bonuses listed above are likely to have people not thinking too hard about that though.
    
    
13. (difference from the modern form) Unless you used the Exhaustion mechanic to force the spell, you don't suffer it as a result of this magic.