Arcanist

Arcanist is a /tg/ slang-term used to refer to a member of any of the various classes in Dungeons & Dragons or Pathfinder (or sometimes other games) that practices arcane magic. This includes, but isn't necessarily limited to, the wizard, the sorcerer, the warlock, the swordmage, the artificer and the bard.

It can also refer to one of two character classes in its own right.

AD&D

The Advanced Dungeons & Dragons version of the Arcanist appeared in the Ravenloft campaign sourcebook for that edition, "Domains of Dread", alongside the Avenger, the Anchorite and the Gypsy. It was a thematic addition to the list of specialist wizard kits, and was joined in that aspect by a version of the Elementalist kit; specifically, arcanists in Ravenloft represent wizards who focus on all manner of lore relating to the denizens of darkness and the creatures of the night. In a way, they're a spookier counterpart to the Bard in the field of the "loremaster class".

Requiring minimum Intelligence and Wisdom of 15+, these uber-specialized wizards gain the following traits:

• Must be a Human or Half-Vistani of Lawful alignment.
• Cannot learn or cast spells from the schools of Evocation, Enchantment, Illusion or Transmutation, which leaves them access to the schools of Abjuration, Conjuration, Divination, and Necromancy.
• Cannot use magic items that draw from the aforementioned schools.
• Suffer a -25% penalty to checks to learn spells that are not in the Necromancy or Divination school.
• Gain a +25% chance to learn spells from the Necromancy and Divination schools.
• Enemies suffer a -2 penalty to saving throws against Necromancy and Divination spells cast by an Arcanist.
• Arcanists gain a +2 bonus to saving throws against Necromancy and Divination spells.
• Can Turn/Command Undead if they were a Cleric of equivalent level.
• 5% chance per experience level to know some relevant fact about a person, object or location that has some significance in the supernatural & macabre history of Ravenloft.
• +2 bonus to all Fear checks and Horror checks.
• Each time an Arcanist gains a level, they must attempt a Dark Powers Check with a failure chance of (5% per level possessed).

Now, you may be thinking that this class looks kind of gimped. Well, there are some big disadvantages, and you need to know a few facts about Ravenloft to understand it:

• Powers Checks, if failed, blight your character with steadily worsening curses, and ultimately cause you to become an NPC if you fail too many of them.
• There is no way to cure yourself of a failed Powers Check.
• Because necromancy is seen as an evil school, you have to take a Powers Check every time you cast a spell from that school.
• In order to preserve the Gothic atmosphere of Ravenloft, divination spells don't work properly here; some spells are outright useless (for example, Know Alignment), some can harm you (telepathy risks driving you insane if the target is something nonhuman), and at best they just have a chance to fizzle out.
• Conjuration spells have a drawback; anything you summon from another world is trapped here when the spell expires. This leaves them free of your control and very ticked off with you. So, casting conjuration spells should be avoided unless it's something you can guarantee will get killed before your control over it ends.

So taking this class is, charitably, a huge challenge, and more realistically will probably get you killed or warped into a villainous NPC pretty quickly. There's reasons why Ravenloft failed to find a particularly deep audience in D&D fans, and the fact that the PCs tend to be so immensely screwed over for the sake of the setting is definitely a big part of that.

Pathfinder

Arcanists are a hybrid class of Pathfinder which merges the magical powers of both Sorcerer and Wizard to force the laws of nature on the table and forcibly violate them while they're screaming. Moreso than plain Wizards, at least.

In a way, they're a variant of Wizards, with the need to memorize spells each day, but they can use a spell multiple times a day since the spell only eats up a slot. Essentially like a Spirit Shaman or a 5th Edition Wizard. On top of that, they also have a Reservoir of innate energy they can use to empower their spells by boosting their DC/Casting Level or for their unique feature: Exploits. See, these either act like spells (like shooting acid out of their palms, making yet another magical shield, or temporarily enchanting weapons) or claim specific aspects of their parent classes (like Sorcerer Bloodline Powers, Metamagic, Wizard Schools, and the like); at level 11 and beyond, you can then select bigger Exploits which boost the Exploits picked earlier, like hijacking spells from baddies on the Arcanist's level or redirecting the directions of certain spells (or absorbing their effects). Their ability to combine damage boosting bloodlines with the energy swapping of the Wizard's Admixture subschool makes them the best blaster in the game (except possibly, Kineticist, who can't do much else).

Archetypes further cross them into other classes, like the Arcane Adept, which taps into the abilities of the Magus like a Bladebound, the Occultist, which does summoning, or the Blood Arcanist, which gives the Arcanist a full Bloodline to gain powers from in exchange for a few Exploits.

Arcanist are a solid Tier 1 class. They've got the same power to smash whatever challenge they're prepped for as a Wizard, just with a slightly different casting system.

Spheres of Power

Strangely, the Arcanist doesn't brutalize game balance nearly as much as its original counterpart did, and partially, this is due to the change in casting mechanics. However, a deeper dive into the class reveals that their core mechanics have also changed. Similar to Sphere Clerics and Sphere Druids, the Arcanist is a full caster with a talent at every even level, so it sounds gimped until one realizes that it has a new mechanic to regain the old flavor of Arcanist. Moldable Talents are flexible talents that can be modified every day. Additionally, they get a stronger version of Consume Spells, as it only costs them 1 spell point to regain three points of Arcane Reservoir. Magical Supremacy lets them use Arcane Reservoir Points instead of Spell Points as needed. While certainly not as attractive as Sorcerer or Wizard, it does gain a degree of daily flexibility neither have in Spheres.

The Classes of Pathfinder 1st Edition
Core Classes:	Barbarian - Bard - Cleric - Druid - Fighter - Monk Paladin - Ranger - Rogue - Sorcerer - Wizard
Advanced Player's Guide:	Alchemist - Antipaladin - Cavalier Inquisitor - Oracle - Summoner - Witch
Advanced Class Guide:	Arcanist - Bloodrager - Brawler - Hunter - Investigator Shaman - Skald - Slayer - Swashbuckler - Warpriest
Occult Adventures:	Kineticist - Medium - Mesmerist Occultist - Psychic - Spiritualist
Ultimate X:	Gunslinger - Magus - Ninja - Samurai - Shifter - Vigilante