Strain
See also: Attunement, Bloodstones
On Tessera, many casters understand magic as interacting with the Weave. See PHB 205 for an introduction to the Weave. However, on Tessera using magic comes at a cost. For both arcane magic and divine magic, interacting with the Weave causes side effects on the caster.
The most conspicuous side effect of most magic is called Strain, which is measured in Strain Points (SP). Immediately after casting a spell, a caster gains SP equal to the spell's level. (Cantrips cause neglible Strain, unless a being casts many in quick succession.) A player is responsible for tracking their character's SP. There is no maximum for how much SP a caster can gain. However, a caster has a Strain Limit, based on their level: (Level+1)*4. For example, a 2nd level wizard has a Strain Limit of (2+1)*4 = 3*4 = 12. However, a 13th level warlock has Strain Limit of (13+1)*4 = 14*4 = 56.
This spreadsheet shows Strain Limits: https://docs.google.com/spreadsheets/d/1t05PH1VAmyEDs2W7DAJOPvIX5Nkscj6BNg5KI_WjIU0/edit?usp=sharing
Innate abilities and similar class features that aren't spells don't cause Strain. This includes abilities and features such as a cleric's Channel Divinity, a druid's Wild Shape, a monk's Stunning Strike, or a paladin's Lay on Hands. In general, if anything includes "cast" in the description, it causes Strain (DM's choice).
A caster can also avoid Strain by invoking one or more Laws of Magic.
Attunement to a magic item creates long-term Strain that can't be purged.
These rules are obsolete. I replaced them with [[1]].
Strain Effects
A caster who keeps SP over their Strain Limit will eventually develop one or more Strain Effects.
The most common Strain Effects are mental. Mild effects can include reduced impulse control, a compulsion, an obsession, or a phobia. Moderate effects can include paranoia, addiction, or depression. Severe effects can include a psychosis--a complete break between the mind and reality.
Physical Strain Effects are rarer. Mild effects can include changes to the caster's eyes, hair, skin, voice, etc. Moderate effects can include a severe allergy (e.g., to sunlight), infertility, or a metabolic need to ingest gold. Severe effects can include substantial and possibly grotesque distortions (e.g., shorter or longer limbs), additions (e.g., a second mouth), etc.
Strain Effects are rarely advantageous. Of course, a caster already suffering from mental effects might view a new physical effect as an improvement. The nature and magnitude of Strain Effects are related to the nature and magnitude of the magic. For example, a caster who gains excessive Strain from casting fear spells might develop a phobia.
The development of one or more Strain Effects reduces the caster's SP, proportional to its magnitude. The rough proportions are Mild (10), Moderate (30), and Severe (50), but the exact effects and proportions are the DM's choice. Once developed, a Strain Effect is a permanent, non-magical phenomenon. For example, Remove Curse won't remove a Strain Effect. Depending on the nature of the effect, mundane or magical means might temper or remove it (e.g., counseling, surgery) (DM's choice).
A caster with Strain over their Limit must periodically make an ability check to avoid developing an Effect. The caster must make this check once per day, and may also need to make when their mental focus is impaired (e.g., by a stressful situation, by exhaustion) (DM's choice). The caster uses their spellcasting ability, including any ability and proficiency bonuses. There is a penalty to the check equal to the SP beyond the limit. Thus, even for a high-level caster the check eventually becomes impossible.
Purging
Most casters learn one or more techniques to purge Strain and reduce their SP. Different orders emphasize and teach different techniques. For example, an order of paladins and clerics might teach purging through prayer, while a warlock may commune with their patron.
As part of a short rest, a caster can purge SP up to (1d6 + Proficiency Bonus). Except for eating and drinking, the caster can't engage in other activities while purging.
As part of a long rest, a caster can purge SP up to half their Strain Limit. Thus, a caster who exactly hits their Limit will need two long rests to purge all SP.
At any time, a caster may be able to purge some SP through appropriate role-play, based on their class, ideals, bonds, etc. A caster's player is encouraged to collaborate with the DM on one-time or recurring means of such purging, which can be idiosyncratic and unique to the the character.
Other Side Effects
In addition to Strain Effects, interacting with the Weave can have other side effects, possibly immediate, of a fixed or random duration, or even permanent. Such effects are the DM's choice.
[Design Notes: I'm trying to achieve two goals with Strain. First, I want spells and magic items to be scarce in Tessera. I want society and industry to mostly operate on medieval technology, not magic. Strain is a strong deterrent to wide-spread, casual use of magic. Second, I want to frame interesting possible role-play and narrative for PC and NPC casters.
Mental illness is a sensitive topic. I know a great deal about it, including personal experience, and I don't treat it lightly. Rather, I want to frame interesting possible role-play and narrative by blending the ignorance, prejudice, and fear around mental illness with how some NPCs (and perhaps some PCs) view magic and casters.]
