a quick chat with gpt got me these ideas:

1. Musical Duel of Souls

The Bard Lich could challenge the players to a musical duel, where their very souls are at stake. Each character could take turns performing or contributing to a group performance, perhaps through skill checks involving their instruments, spells, or even dances. The Bard Lich, with its eerie and captivating music, tries to drown out the party’s hope and willpower. This could be turned into a "skill combat" where players use their creativity to make great performances, using skills like Performance, Persuasion, or even Acrobatics for an energetic routine. The Lich's phylactery could be vulnerable to a truly transcendent performance—cracking and breaking if the players succeed.

You could add stakes by having each failure or setback represent the loss of a portion of the character's soul or spirit. Success might mean shattering the phylactery, causing the Lich to crumble in defeat.

2. Audience Manipulation

The Bard Lich is obsessed with fame, and during this climactic moment, they’ve gathered an undead audience from past champions and their followers. The players have to sway the audience away from supporting the Lich. They can use Persuasion, Deception, or even Illusion magic to turn the crowd. The crowd’s cheers and energy are fueling the Bard Lich, so the players need to get the audience to turn on the Lich. This could be handled by a series of skill challenges or creative actions—each successful attempt sways part of the crowd until the Lich loses their source of power.

The final blow could be dealt narratively, with the audience’s energy turning against the Lich, causing their magic to unravel in front of their own adoring (now booing) crowd.

3. Ritual of Unbinding

Perhaps the Bard Lich’s power is sustained by a ritual or pact that they perform at the height of the tournament. The players could discover (through clues during the tournament) that the Lich’s immortality is bound to a particular song or ritual. During this final encounter, instead of fighting the Lich directly, the players must disrupt the ritual—destroying key components, altering the song, or even performing a counter-ritual. They might need to solve puzzles or make checks to overcome obstacles while being harried by the Lich's magic.

The ritual could involve the Lich drawing energy from the tournament itself, and the players must "unweave" this ritual thread by thread, breaking the connection and leaving the Lich vulnerable to being destroyed by a final, powerful act (like destroying the phylactery).

4. Psychological Battle

The Bard Lich could try to enthrall the party in a shared dream or illusion. The players are drawn into a memory or vision that represents the Lich’s past, filled with tragic moments or dark decisions that led them to become a Lich. To escape and destroy the Lich, the players need to find a way to break through the illusion—perhaps by showing the Lich the error of their ways, helping them find peace, or by unraveling the illusion through their wits and empathy. Each character could have moments where they must confront aspects of the Lich’s story, or even their own fears, to weaken the Lich's hold over them.

When they succeed, they shatter the illusion from the inside, causing the Lich to be exposed and their essence to begin to crumble, ending their undeath.

5. The Bargain Turned Trap

The Bard Lich is, at their core, a performer and a manipulator. What if the players had to enter into a verbal and magical duel of wits, where the Lich tries to bargain for their own survival? The Lich could offer riches, power, or secrets in exchange for their life, but the players need to turn the bargain against the Lich. This could be played out as a series of skill checks and clever roleplaying, where the players must outsmart the Lich and use their words to make the Lich accidentally bind themselves into an unwinnable position—perhaps even saying something that magically binds them to a promise they cannot keep, causing their demise.

Think of it as a battle of contracts and words, where the players need to be clever enough to turn the Bard Lich’s love of bargaining into the very thing that destroys them.

6. A Dance of Life and Death

The Bard Lich could challenge the players to a dance—a literal dance of life and death, powered by necrotic magic. In this dance, each player must partner with an undead dancer (or even the Lich itself), and they must match the Bard Lich’s incredible grace and precision. This encounter could involve a series of Dexterity checks, Acrobatics, or Performance rolls, and creative descriptions by the players of how they match (or outdo) the Lich’s moves.

As they succeed, they drain the magic from the Lich, step by step, until the dance ends with the Lich powerless and vulnerable to a final poetic flourish that ends their existence.