There are twelve illusionist spells of 3rd-level in AD&D, the same number as their were in OD&D. Phantasmal killer was on this list in OD&D, but has been bumped up to 4th level. Dispel exhaustion has also been bumped up to 4th level. Replacing them are rope trick and dispel illusion, the latter of which has been bumped up from 2nd level.
Continual Darkness: Like the 2nd-level magic-user spell darkness 15' radius it creates a 3" diameter globe of impenetrable shadow. It has a flat range of 6" (vs. 1"/level for m-u) and a casting time of 3 segments (vs. 2 for m-u). The biggest difference is that this version of the spell is permanent in duration.
In OD&D, this spell was based on the anti-cleric's continual darkness, which was a reversed version of continual light. That gave it a whopping diameter of 24", which has been greatly nerfed here.
Continual Light: Like the 3rd-level cleric spell, it creates a globe of light that illuminates a 6" radius. It has a range of 6" (vs. 12" for clerics) and a casting time of 3 segments (6 for clerics). Both versions are permanent.
As mentioned above, the biggest difference from the OD&D spell is that the illumination has been greatly reduced from 24".
Dispel Illusion: The caster is able to automatically dispel any phantasmal force created by a non-illusionist. It can dispel any illusion/phantasm created by an illusionist, with a base 50% chance adjusted up or down depending in the level difference between the two casters.
In OD&D, this was a 2nd-level spell. It's able to dispel any illusions cast by a non-illusionist, which is a broader rule and probably a better one; the AD&D version doesn't account for new spells that may be introduced. Against those of other illusionists it uses the OD&D rules for dispel magic, which was a ratio of the dispeller's level over that of the original caster.
Fear: Like the 4th-level magic-user spell, it causes creatures caught within a conical area to flee in panic. The only difference is that it doesn't require any material components, and it has a segment time of 3 (vs. 4 for magic-users).
The OD&D spell worked similarly, but targets had a flat 50% chance
to drop what they were carrying, unmodified by level. The duration was 6
turns, whereas in AD&D targets will run for a number of rounds
equal to the caster's level. The spell had a range of 24", while the
AD&D spell only goes to 6". The OD&D spell functioned like the fear wand,
though, so it's still a 6" cone. Does this mean that OD&D casters
could designate the beginning of the cone anywhere within 24"? It feels
a little odd, but I guess it's not that different from lightning bolt. The AD&D spell almost certainly originates from the caster. (Yeah, I did a a cut-and-paste from when I covered the magic-user version. I have a life to live, and I'd do it again in a heartbeat.)
Hallucinatory Terrain: Like the 4th-level magic-user spell, it can changes the appearance of terrain in a 4"x4" area (with 1" added to both dimensions per level). The magic-user spell was a flat 1" square per level. The illusionist version also has a 2" bonus to range over the magic-user version. The illusionist spell has a casting time of 5 segments (vs. 1 turn for magic users, which is a lot longer).
The OD&D spell is similar, if a bit more vaguely worded. It
doesn't have a specific area of effect, it's simply said that it affects
a "large area". It had a range of 24", whereas AD&D has as range
of 2" + 2"/level. (See, I told you I'd do it again.)
Illusionary Script: Gary, the word you're looking for is "illusory". Anyway, the caster can write something and then make it appear as though it's written in a foreign language. Only those that the caster designates can read it, with all others being confused for 5-20 turns. This duration drops by a turn per level of the victim. Other illusionists can tell that the script is an illusion. The material component is a lead-based ink that must be specially made by an alchemist.
The OD&D spell only confused its victims for 1-6 turns, not affected by their level. It also had a limit of one page of writing, which isn't present in AD&D.
Invisibility, 10' Radius: Like the 3rd-level magic-user spell, it casts an invisibility spell on every creature you want within 10 feet of the original target. The only difference is that the illusionist spell requires no material components.
The AD&D spell isn't clear as to whether it grants invisibility to multiple creatures within the area, or if it creates an area of invisibility that moves with a single target. I've gone with the former option. The OD&D spell was similarly vague. It had a range of 24", whereas in AD&D it's a touch spell.
Non-Detection: Makes the caster completely immune to detection spells, ESP, clairaudience, clairvoyance as well as scrying from magic items. It's a somewhat advanced version of misdirection, although by providing false information the latter spell remains useful. Its material component is a pinch of diamond dust.
The OD&D spell worked similarly, although it wasn't as specific about which spells it protected against. It did specify protection against ESP and crystal balls. It had a duration of 2 rounds/level, whereas the AD&D spell lasts for 1 turn/level, about five times longer.
Paralyzation: The spell creates an "illusionary muscle slowdown" in its targets, paralyzing them if they fail a saving throw. It affects a 2" x 2" area, but can only work on a Hit Dice total up to double the caster's level. A dispel magic or dispel illusion will end the paralysis, or it can be ended by the caster. Otherwise, it apparently lasts forever. Harsh.
The OD&D spell worked the same, although it didn't limit the spell area at all. It had a range of 18" (vs. 1"/level in AD&D).
Rope Trick: Like the 1st-level magic-user spell, this causes a rope to stand on end, and opens a portal to a pocket dimension at the top. The only difference is that it takes 3 segments to cast, instead of 2 for magic-users.
The OD&D spell could work with a shorter rope, but it could only let the caster and three others inside the pocket dimension; the AD&D version allows five others inside. It had a duration of 6 turns +1/level, whereas AD&D gives it 2 turns/level.
Spectral Force: This spell functions like phantasmal force, but along with the visual element it also includes sound, smell and heat/cold. It lasts for 3 rounds after the caster stops concentrating on it.
The OD&D spell was called spectral forces. It was otherwise similar, with one major difference: it created illusions that could not be destroyed by touching them. This isn't specified in AD&D. Also, it lasts for 5 "turns" after concentration, as opposed to AD&D's 3 rounds.
Suggestion: Like the 3rd-level magic-user spell, the caster is able to influence a single target. The illusionist spell lasts for 4 turns + 4/level, whereas the magic-user version lasted longer at 6 turns + 6/level.
The only real difference that the OD&D version of the spell had was its duration of one week.