Recently, I wished to plan a module for my PCs that would expose them to the horrors of being possessed by a powerful undead. Unfortunately, the magic jar rules found in the Player's Hand Book (PHB) are designed for quick play, and I wanted this particular possession attempt to drag on for at least a week.
     This way, I could build an entire module out of the characters' attempts to rid themselves of the possessing entity. The rules outlined in the PHB would have had the possession attempt end too soon. I wanted the possession attempt to drag on and on, making the expulsion of the evil mage a difficult goal to accomplish, but still possible.
     The methods I have outlined below allow for a slow, drawn out process by which an evil mage might attempt to possess a PC. The process takes time, and may stretch out over weeks or even months. It is therefore not meant to replace the standard magic jar rules, but instead work as a substitute. It is especially nice for those players who know the regular rules inside and out.

Step 1:     Find the attacker's AR (Attack Rounds). To do this, divide the attacker's Wisdom by two, rounding down. This number represents the number of rounds the attacker has to conduct his attack.

Step 2:     Find the defender's DT (Defense Threshold). To do this, divide the defender's Wisdom by two, rounding down. This number represents the gradients of the defender's psyche (a defender with a Wisdom of 12 would have 6 gradients to his psyche).

Step 3:     Determine the attacker's and defender's DRB (die roll bonus). This number is equal to the individual's level, divided by four (round down).

Step 4:     Begin the attack.

Attack

     In order for the attacker to defeat the defender, and usurp control of the defender's body, he must overcome the defender's psyche (represented by the defender's DT). The attacker has a number of rounds equal to his AR to accomplish this. To begin, the defender possesses all of his DT's.

Attack:     The attacker rolls 1d20, adds his DRB, and compares the result to the defender's Wisdom score.

Defend:     The defender rolls 1d20, adds his DRB, and compares the result to the attacker's Wisdom score.

Compare:     Whoever beat the other person's Wisdom score by the most has won this round, and possesses one of the defender's DTs.

     This process is repeated, once for every AR the attacker has (e.g., if the attacker had a Wisdom of 16, then he would have 9 ARs, i.e., 9 rounds to conduct his attack. If the attacker takes over all of the defender's DTs before the full number of AR's have expired, then the possession attempt is still a success, and the battle ends early.
     At the end of the last AR, the successes are compared. If the attacker has taken over none of the defender's DTs, then the possession attempt has failed. The attacker then rolls a Wisdom check. If he succeeds, he goes back to his phylactery, and is free to try to possess another body. If he fails, his psyche evaporates, and he is forever dead.
     If the attacker has taken over all of the defender's DT's, then the possession attempt has been a success. The attacker possesses the defender's body. The defender's psyche may manage to inhabit the phylactery. The defender rolls a Wisdom check, with a -4 penalty (the defender is unfamiliar with the phylactery). If he succeeds, then the defender has inhabited the phylactery. He may later attempt to possess someone else, but due to the fact that he is unfamiliar with the phylactery, he must first rest for one week.
     If the defender fails the Wisdom check, then his psyche is destroyed, and he is forever dead.
     If either side rolls a 1, they lose automatically, and immediately forfeit a DT. On the other hand, if either side rolls a 20, they win automatically, and gain a DT. If both roll a 1, or both roll a 20, then the round is a tie, and the DT status does not change.
     Note, this only applies on a roll of a natural 20 or 1. Scores that, after adjustments, equal or exceed 20 do not apply to this rule. If one side rolls a 1, and the other rolls a 20, DT loss and gain are cumulative (the winning side would go up 2 DT's).

Split Successes:     If the DT's are split between the attacker and the defender, then a special situation arises. The attacker is partially successful, and his spirit inhabits the defender's body. However, he is not yet in complete control.
     Each morning, upon rising, the attacker and the defender each roll a Wisdom check (with a +1 bonus for each DT that individual possesses), and add their respective DRBs, with a target number of their opponent's Wisdom. The winner gains another DT, and the loser, naturally, loses one of his.
     The winner of this morning battle is also in control of the body for the day. The loser can take no action, and is merely a passenger in the body.
     If this daily battle results in one side winning all the DTs, then the battle is over. The winner possesses the body, and the psyche of the loser is cast out. The loser's psyche is weakened from the constant battling, and cannot attempt to take over another body for a period of one week.
     Note that this process could take weeks, months, even years. It is a true challenge for the player to role play this situation. For that reason, the DM may wish to alter the rules, so that either the attacker or the defender must merely win a majority of the DTs to gain possession of the body.
     Spells such as heal have no effect. The invading psyche is so closely bound to the psyche of the defender that only the most powerful of magics could possibly separate them. Limited wish will allow the defender automatic control of his body for one day, but will not gain him back any DT's. Wish will completely expel the attacker or the defender, depending on whom the wish favors.

Sample Battle:     Par, an evil 10th level cleric, is stuck in his phylactery. Crane, a 5th level thief, enters the room where the phylactery is hidden. Par's Wisdom is 17, and Crane has a Wisdom of 9.

1. First, we determine the Par's AR.

     17/2 = 8.5, which rounds down to 8. Therefore, Par has 8 rounds in which to conduct his attack.

2. Next, we determine Par's DRB.

     10/4 = 2.5, which rounds down to 2. Therefore, Par has a DRB of 2.

3. The next step is to determine Crane's DT.

     9/2 = 4.5, which rounds down to 4. Therefore, Crane's psyche can be broken down into 4 parts.

4. After this, we determine Crane's DRB.

     5/4 = 1.25, which rounds down to 1. Therefore, Crane has a DRB of 1.

     We then conduct the attack. Remember, Par has 8 rounds to conduct the attack.

Round One

Par rolls a 17, and adds his DRB (2), with a result of 19. This beats Crane's Wisdom by 10. Crane rolls a 15, and adds his DRB (1), with a result of 11. This fails to beat Par's Wisdom. Therefore, Par gains a DT.

Result: Par has 1 of Crane's DTs, Crane has 3.

Round Two

Par rolls a 6+2=8, failing to beat Crane's Wisdom. Crane rolls a 19+1=20, beating Par's Wisdom by 3. Crane gains a DT.

Result: Par has 0 of Crane's DTs, Crane has 4.

Round Three

Par rolls a 5+2=7, failing to beat Crane's Wisdom. Crane rolls a 16+1=17, matching Par's Wisdom. Crane gains a DT. Since Crane has all the DTs, the results stay the same.

Result: Par has 0 of Crane's DTs, Crane has 4.

Round Four

Par rolls a 7+2=9, matching Crane's Wisdom. Crane rolls a 10+1=11, failing to beat Par's Wisdom. Par gains a DT.

Result: Par has 1 of Crane's DTs, Crane has 3.

Round Five

Par rolls a 17+2=19, beating Crane's Wisdom by 10. Crane rolls a 18+1=19, beating Par's Wisdom by 2. Par beat Crane's wisdom by a number greater than that than which Crane beat Par's .Par gains a DT.

Result: Par has 2 of Crane's DTs, Crane has 2.

Round Six

Par rolls a 13+2=15, beating Crane's Wisdom by 6. Crane rolls a 2+1=3, failing to beat Par's Wisdom. Par gains a DT.

Result: Par has 3 of Crane's DTs, Crane has 1.

Round Seven

Par rolls a 1, failing to beat Crane's Wisdom. Crane rolls a 8+1=9, failing to beat Par's Wisdom. Par loses a DT. A roll of 1 is an automatic failure, and results in the immediate loss of a DT.

Result: Par has 2 of Crane's DTs, Crane has 2.

Round Eight

Par rolls a 15+2=17, beating Crane's Wisdom by 8. Crane rolls a 2+1=3, failing to beat Par's Wisdom. Par gains a DT.

Result: Par has 3 of Crane's DTs, Crane has 1.

Overall Result

      Par has captured 3 of Crane's 4 DTs, but is not in control yet. Crane remains in control of the body for the rest of this day, with Par's psyche along for the ride. The next morning, they each contest their wills again:

Par rolls an 2+2=4, failing to beat Crane's Wisdom. Crane rolls a 16+1=17, matching Par's Wisdom. Crane gains a DT.

Result: Par has 2 of Crane's DTs, Crane has 2.

      For the rest of this day, Crane is in control of the body, and Par can only observe. This process repeats the next day:

Par rolls an 15+2=17, beating Crane's Wisdom by 8. Crane rolls a 10+1=11, failing to beat Par's Wisdom. Par gains a DT.

Result: Par has 3 of Crane's DTs, Crane has 1.

     This time, Par is in control for the day, and Crane is but a helpless observer. This process will repeat until one side or the other is in possession of all the DTs, at which point the other psyche is repelled from the body. If at this time, the phylactery still exists, the expelled psyche may attempt to move into it, by passing a Wisdom check (with a -4 penalty if having never been in this phylactery before). The attempt is made with a -1 penalty for every 25 miles between the expelled psyche and the phylactery.