Dark magic is any warlock spell or spell-like ability, or any spell from the school of Necromancy or any spell or magical ability which manipulates the mind of another against their will (charm, compulsion, fear effects primarily).
The perspective gained through manipulating life force and the minds of sapient creatures provides a greater understanding of magic. However, it also separates the caster from the greater whole: a practitioner of dark magic gradually loses their ability to empathize with other people. This is represented as a gradual shift in alignment.
Warlock powers are a bit different. Their come from an advanced, alien understanding of metaphysics. The use of warlock magicks does not corrupt the caster directly; rather, the alien mindset necessary to warlock abilities makes them susceptible to thinking of themselves as superior to non-warlocks. A warlock who abuses their powers for control over the minds of others, is gratuitously cruel, or otherwise acts callously or sadistically is subject to the same loss of empathy as traditional casters.
As a caster becomes more corrupt, their ability to channel dark magic improves. When a corrupt character uses necromancy, compulsion, or warlock spells (but not spell-like abilities), they may apply metamagic effect(s) worth X sorcery points to that spell for free, subject to the normal restrictions for such effects, where X is equal to their corruption tier (1-3). Corrupt casters can use this free metamagic in addition to their normal metamagic mechanic, effectively gaining 1-3 free sorcery points every time they cast such a spell (with the caveat that these free points can only be used for metamagic).
Each tier also grants a free virtual metamagic feat which can only be used in this way. The character must meet the prerequisites for the feat, and this virtual feat cannot be used to fulfill prerequisites for other feats. If the character ever gains such a virtual feat legitimately, they should immediately after choose a replacement which they qualify for.
Each time a PC’s corruption tier increases, their alignment shifts one step towards evil or chaos (player choice); for example, a lawful good character could become lawful neutral or neutral good. A chaotic neutral character would become chaotic evil. At tier 3, PC’s become NPC’s.
The tiers should be treated as different stages of addiction: Tier 1 is a habitual but not compulsive user. Tier 2 is a compulsive user but still functional, and at tier 3, the addict is no longer in control of their actions.
It is possible to move in either direction, though kicking the habit should be slow and the product of long term, dedicated in-character effort. Additionally, once a character has descended to a given tier, it becomes much easier for them to sink back down in the future if they do manage to pull free (achieve a lower tier through roleplaying). At GM discretion, a character’s alignment might change if that character shows appropriate contrition. This doesn’t have to be a direct reversal of the original changes resulting from a corrupted spirit; a penitent character might see things differently after repenting, resulting in an alignment other than the one they originally lived by.
In plain language, when applied to players this system is about risk vs. reward. A player might choose to abstain from dark magic entirely; they might choose to dabble, doing their best to stay Tier 1; or they might risk losing it all in the pursuit of power, attempting to maintain a Tier 2 status. A Tier 1 character who goes a little too far can recover; a Tier 2 character who loses control does not have that option, as at Tier 3 the player loses control of their character permanently.
It should be noted that this system applies to NPC’s as well as player characters; practitioners of the forbidden arts should be approached with great caution. Imagine, for example, an evil sorcerer who can quicken spells at will.