AD&D provides ten 5th-level spells for clerics, up from the six that were in OD&D. Carrying over from the previous edition we have commune, dispel evil, insect plague, quest and raise dead. Create food was a 5th-level spell in OD&D, but in AD&D got shifted down to 3rd level. Atonement, cure critical wounds, flame strike, plane shift and true seeing are all-new.
Atonement: This spell, new in AD&D, grants atonement to any character for deeds done against their will, i.e. when under mind-control or some other outside influence. It doesn't work on deeds done deliberately, or those that the character has no remorse for. It also removes the effects of magical alignment change. It's a niche spell, but very useful when said niche must be filled, especially for paladins and clerics. (I guess this is confirmation that the gods - or whatever entities or forces govern the universe - recognise the difference between acts done unwillingly, and those done willingly, and that they value repentance.)
Commune: The cleric contacts their deity, or the deity's agents, and can ask a number of "yes or no" questions that are always answered correctly. The DM is encouraged to put a limit on the amount this spell can be used, as the gods get annoyed with this sort of thing (probably given concrete rules in the Dungeon Masters Guide).
The implication of this spell is that all of the gods are all-knowing; how else would they have the answers to any possible question a cleric might ask? I'm not a fan of this, as I prefer the gods to be more fallible than that, as they often are in mythological tales.
In OD&D, clerics casting this spell were allowed three questions, but here that number is now one per caster level. In OD&D, the spell was limited to one use per week, as opposed to AD&D's nebulous guidelines. It also had a special "one time a year" casting that allowed double the normal number of questions, which has been cut out of the AD&D version completely.
Cure Critical Wounds: The next step up in the healing spell hierarchy, this spell heals 3d8+3 hit points. It can also be reversed to cause damage. I'm surprised that this is debuting so late; OD&D was really lacking in high-end healing spells.
Dispel Evil: Causes any evil summoned creature to return to its home plane, or place of origin. It lasts for 1 round per level, and is delivered with touch, so I wonder if multiple creatures can be so dispelled? While it's in effect, evil summoned creatures suffer a -7 penalty to hit the caster.
It also says that it works on "monsters enchanted and caused to perform evil deeds". Does that mean characters that have been charmed? I'm not sure how that would work. I can understand applying the to-hit penalty, but should they be banished to their home plane? What if they're from the Prime Material? Would the spell banish a possessing entity, or dispel the charm? It's all a bit vague.
The OD&D version of the spell was a higher-level version of dispel magic, that worked on any "evil sending or spell". In effect, it reads to me like a completely different spell. Oh yeah, it was originally written as dispell evil, but Gary learned some grammar and spelling between editions.
Flame Strike: A very rare damage-dealing cleric spell makes its debut here. It creates a column of flame that deals 6d8 damage (save for half). Doesn't get much more basic than that, does it?
Insect Plague: Creates a swarm of insects that obscures vision and deals 1 point of damage per round to anyone within the area. It also causes all creatures of less than 5 Hit Dice to check morale or flee, while those of less than 2 Hit Dice flee automatically. Mostly the spell seems useful for frustrating spellcasters and dispersing low-level opponents. Smoke will drive insects away, and most area damage spells will temporarily clear a space.
The OD&D version of the spell was very different. Firstly, the caster could direct the insects, whereas in AD&D the spell is stationary. The OD&D spell had an area of 36 square inches, while AD&D gives it a 36" diameter. The spell didn't deal any damage in OD&D, but it automatically routed creatures of 3 Hit Dice (while seemingly having no effect on those higher). AD&D dropped the range from 48" to 36". Lastly, in OD&D the spell could only be used above ground, which might be the biggest difference of all.
Plane Shift: This spell can be used to transport the caster and up to seven others (or maybe six, it's not super-clear) to another plane of existence. The material component is a forked metal rod, with the size and type of metal determining which plane is reached. (This is another thing I expect to be explained in the DMG.) It can be used as an offensive spell as well, if you feel like sending your enemies to Hell or something.
Quest: Ah, quest, the favoured tool of tyrannical DMs everywhere. The victim of this spell is forced to undertake some task for the cleric, and for every day that they avoid the task they suffer a cumulative -1 penalty to all saving throws. I've never seen this one used by PCs, as in my experience players like to do important tasks themselves. As alluded to above, I've mostly seen it used by DMs to force players into an adventure. If you said that the game would be better off without this spell, I wouldn't disagree with you.
The major difference between this and the OD&D version of the spell is that OD&D clerics could curse those who avoid the quest with pretty much anything they wanted. Sometimes OD&D went a little too far in leaving things to the DM.
Raise Dead: Can raise the following creatures from the dead: dwarves, gnomes, half-elves, halflings and humans. Not monsters, not animals, and most notably not elves and half-orcs. No explanation is given here for why those races can't be raised; I'm pretty sure that later on it's explained that they have no souls, but at this point it's a mystery.
The spell can also be reversed as slay living. (I've never considered this before, but elves and half-orcs should be immune to the reversed spell as well.) Good clerics are warned against indiscriminate use of the spell, generally only being allowed to use it on people they know are super-evil.
In a neat touch, raise dead can be used on newly created undead (as when used on a regular corpse, it can be dead no longer than 1 day per caster level). It won't work on skeletons, though, because the spell doesn't restore lost body parts. (It's not stated, but it probably shouldn't work on any incorporeal undead either.)
Looking at the OD&D version of the spell, I'm surprised to see that it worked on Men, Elves and Dwarves, and excluded Hobbits. That's going to be a hard one to reconcile. Looking back at the post where I first covered this spell (waaaay back in 2009) I took a stab at it thusly: "I'll tie this into the waning of the elven peoples - as they grow
weaker in life, so they are drawn ever more strongly to the realm they
reside in after death. With Hobbits and other creatures, I'm theorizing
that it actually takes contacting the land of the dead to return the
departed soul to life - wherever hobbits go when they die hasn't been
discovered yet." That's a decent start, and allows me to explain how this spell will work on other creatures later on as well. Alternatively, I could tie it in to the 3rd edition idea that dead creatures have to want to come back in order to be raised successfully. If Hobbits are content in the afterlife, it's very probable that they'll want to stay there unless they are the rare adventurous sort.
True Seeing: This spell allows the caster to see things as they really are. Not only does it see through illusions, but it reveals invisible, ethereal and astral creatures, as well as secret doors. It even lets the caster see a creature's "aura", revealing their alignment. Even polymorphed creatures will be seen in their original form. The spell can be reversed, and the target will see things as their opposites.
The material components for this spell ensure that it won't be overused. It requires an eye ointment made from mushroom powder, saffron and fat that must be aged for 1-6 months. (The reversed spell instead uses ointment made from oil, poppy dust and pink orchid essence.) The ingredients have no value listed, but the time needed is the real factor in limiting this spell.