I made a thing.

Spoiler: Fantastic Mythos
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Bargain of Ruin
Prerequisite: Privacy Inhibiting Attachment
Your capacity to be both boon and bane to those who wield you deepens further, as you draw even from your wielder to fuel yourself.
You may, as a full round action, engage or disengage a Boon and Bane, both selected from the Basic Manifestations of this Mythos you possess. You must select one of each, but the Boon and Bane need not come from the same Manifestation. You may only have one Boon and Bane active at the same time.

Your wielder is not necessarily aware of the Boon and Bane, or their effects, until it is too late. Many are the weapons who have taken advantage of this, leading their wielder to ruin just as they come across a more worthy owner, or indeed subverting their wielder until they are little more than a wandering meat-puppet of the Syntrofos, bearing it to battle for its own ends.

Basic:
Blood-letting Fury: Boon: Your edge sharpens, your swings cut deeper, and your blows strike harder. You deal an extra amount of damage equal to twice your Charisma Modifier, minimum 1. (Select the amount when you activate the Boon).
Bane: Whilst wielding you, your wielder is in a Rage:

Spoiler: Rage Effects
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The effected character may not show any manner of restraint in their attacks (for example, they may not voluntarily deal nonlethal damage if there is a suitable lethal method available, or purposefully pull their punches to deal less damage). They may not voluntarily perform movement that would take them farther away from all of their enemies (they may switch targets, but not forsake all of them). If a foe is dropped unconscious, and there are other dangers afoot, they may focus on those other dangers, but if there are no other enemies, dangers, or pressing concerns, they must attempt to kill fallen enemies. Any creature that strikes them (even if that creature is under some kind of outside influence) is considered an enemy (and cannot be considered an ally, obviously) for the duration of the Rage.

A Raging character takes a -4 penalty on all skill checks (other than Balance, Climb, Intimidate, Jump, Listen, Sense Motive, Spot, Survival, Swim, and Tumble checks), and ranged attack rolls (except with thrown weapons), and cannot cast spells, manifest psionic powers, or utilize Truenaming or Shadow Magic (Binding, Incarnum, and Invocations are fine).

Additionally, your wielder takes your Charisma Modifier in damage each time they make an attack with you.

Folly of Fortune: Boon: Luck is a fickle beast, but you seem uncannily blessed. While your wielder holds you near, they gain a luck bonus on all attacks and skill checks equal to half your charisma modifier.
Bane: The flame that burns twice as bright burns half as long. While your wielder holds you, they age twice as quickly. Additionally, they suffer a penalty to all saves equal to half your charisma modifier.

Two Faced Plague: Boon: In the right light, you seem to glisten with an unhealthy looking ichor. Once per round, when your wielder deals damage with you, you deal 1 point of ability damage to a creature you wounded. Select the ability to damage when you activate this Mythos.
Bane: Your wielder takes on an... unhealthy look. Whilst your wielder holds you, they suffer one negative level. This never results in permanent level loss, but cannot be healed until the wielder has spent at least 1 week without you (which is difficult, considering that you are cursed). Additionally, once per round, when your wielder deals damage with you, they suffer one point of ability damage to a score of your choice.

Extremes of Grace and Laxity:Boon: You strike more swiftly, more surely, than other weapons. Once per round, after dealing damage, you may choose to increase you and your wielder's initiative by 1. If you do not manage to damage anything, however, your initiative returns to its original value.
Bane: Your wielder becomes more susceptible to the whims of others. They take a penalty on all opposed checks equal to your Charisma Modifier. Additionally, their alignment will, over time, shift to match your own. This effect can slowly reverse if the wielder is separated from you for at least a week.

Advanced:
Corruption Through Variety: You gain one Basic manifestation of this Mythos that you do not already possess.


Literally the first bit of homebrew I have made. Do please point out if the balance is all horribly wrong.