I don't have much experience using this spell, and I want to explore some options aside from the cheesy pixie-polymorph combo. I'll be rating them on a 5 star system.
CR 1/4, 8 creatures summoned.
Blink Dogs **
their damage is pretty bad. They have a teleport. Maybe you could do something cool with using them for mounts if you have a party full of goblins, gnomes, or other small creatures that can ride medium creatures. Overall, not very useful.
Sprites *****
Now, these are kind of interesting. Their HP and damage are abysmal, BUT they use poisoned arrows that cause Poisoned for 1 minute on a DC 10 CON save and Unconscious on a DC 6 CON save. They also have a fly speed of 40 and can turn invisible at will (+8 stealth). I think you could do some really creative things with sprites. If you have an archer in your group, you could have them share their poisoned arrows (obviously subject to DM approval). On their own, they could still do some cool stuff such as help you silently infiltrate a camp. Even with +3 con, there's a 73% chance that at least ONE of those arrows will cause the mob to fall unconscious.
Sprites also have an ability where they can determine a creature's alignment. Maybe situationally useful.
Mite *****
Similar to Sprites, Mites have terrible HP and damage (better than sprites though), but have a really interesting trick up their sleeve. Mites can place a hex on a creature. The hex has a recharge of 6 and there is no save. It can be done while hidden without revealing the mite's location. The hex is quite powerful: "While the creature is affected by this hex, whenever it makes an attack roll, an ability check, or a saving throw, it must roll a d6 and subtract the number rolled from the d20 roll". The fact that you can summon 8 of these things and hex a whole group of enemies is pretty incredible. The hex has a strange stipulation that when it rolls the d6, the creature can attack an ally to ignore the effects of the hex until the end of its next turn.
CR 1/2, 4 creatures summoned.
Satyr *
The only thing these offer is damage. You'd be better off with conjure animals.
Darkling **
These are mildly interesting. They have a feature similar to sneak-attack that requires advantage. If you can set that up, their damage isn't that bad (1d4+2d6+3). Light sensitivity means they are limited in daylight though. When killed, they let off a bang that can blind enemies for 1 turn (DC 10 con save).
CR 1, 2 creatures summoned.
Dryad *****
Dryads are pretty dope. They are spellcasters. Each one gets 3 casts of Entangle and Goodberry as well as 1 cast of Pass Without Trace. You can get 60 points of healing from the goodberries (will take a few minutes for people to much them down of course), which about matches what you could get from a 4th level cast of Healing Spirit. Dryads are perfect for summoning prior to sneaking off somewhere. You can get covered by PWT for free, get your 60 goodberries, then the Dryads can attempt to Entangle enemies when you start combat.
Dryads also have a few other tricks up their sleeves. They can speak with beasts and plants, so you might be able to get some information from nearby trees. They have Tree Stride, making them extremely mobile in a forest. And they also have a Fey Charm ability that allows them to charm 1 humanoid or up to 3 beasts. Imagine being in a forest. You summon your dryads. Then you send them off to gather beasts to help you out. They each return with 3 wolves under their charms. Nice, right?
CR 1, 2 creatures summoned.
Nereid****
This creature has some pretty nice abilities. Drowning Kiss (Recharge 5-6) is a con save with nice acid damage and renders the target unable to speak on a fail. Could be very useful for targeting enemy casters. Blinding Acid is an attack that... can blind a target on a hit. It can also cast Control Water, including using it to create a whirlpool for up to a 30 ft cube of water.
Naiad**
This creature is only really useful for its spellcasting which includes a single cast of fly and hypnotic pattern as well as 3 casts of phantasmal force. Not that great, but it's a roundabout way to get access to those spells which might be useful in the right situation.
Note: By design, the DM chooses what actually appears, the player only chooses the number/CR. [SageAdvice]
Something to consider about Blink Dogs:
Their Teleport action includes a free bite attack, so they can run in as a mob, get 8 attacks with advantage (flanking), and then teleport 40ft out of melee/Aoe range without provoking Attacks of Opportunity.
With a 1 hour duration, you might want to take advantage of 8x perception checks with advantage. These are intelligent creatures who can look for traps, treasure, and communicate important information non-verbally. They can also fetch and bring things of value back to you.
Properly run, Blink dogs can be very effective. Just depends on where you are and what you need.
Note: By design, the DM chooses what actually appears, the player only chooses the number/CR. [SageAdvice]
Something to consider about Blink Dogs:
Their Teleport action includes a free bite attack, so they can run in as a mob, get 8 attacks with advantage (flanking), and then teleport 40ft out of melee/Aoe range without provoking Attacks of Opportunity.
With a 1 hour duration, you might want to take advantage of 8x perception checks with advantage. These are intelligent creatures who can look for traps, treasure, and communicate important information non-verbally. They can also fetch and bring things of value back to you.
Properly run, Blink dogs can be very effective. Just depends on where you are and what you need.
True, it's ultimately up the the DM what creatures you summon. I feel like most good dm's will consider your request and at the very least not give you completely useless summons. Here is the relevant part of that Sage Advice answer
A spellcaster can certainly express a preference for what creatures shows up, but it’s up to the DM to determine if they do. The DM will often choose creatures that are appropriate for the campaign and that will be fun to introduce in a scene.
For blink dogs, IMO they compare pretty poorly with wolves. Conjure Animals can summon the same number of wolves which do about twice as much damage and have pack tactics (a lot of games don't do flanking). Blink Dogs do have better HP and the blink ability. They aren't useless, but they aren't good either IMO
A Shark compares pretty poorly to a Rat, if you compare it by how quickly it can run on land. :)
In a typical encounter, creature actions are often simplified to a fairly finite set of behaviors. It's true that if limited to mob behavior, the wolf will probably do more damage over the course of 6 rounds. However, if played in a dynamic world with clever players, the Blink Dog is effectively an NPC, rather than an animal. It has human levels of intelligence, understands two languages, and can take initiative if given latitude ("Kill them" or "help us", as opposed to "bite them").
A wolf might bite for 2d4+2 damage, but a Blink Dog could teleport onto a ledge and knock over a statue/boulder to deal massive damage or change the battlefield. A wolf might get advantage via Pack Tactics, but a Blink Dog could trigger traps, snuff out lights, and understand resistances and vulnerabilities.
If your party were locked in a cell, you could tell the Blink Dogs "Get Help", and they would understand that getting help could either mean finding an ally, or tracking down a guard with keys and dragging them back to you. When they return, they might volunteer information that you didn't think to ask for, such as the number of guards and the fastest way to the outside.
Having 8 teleporting NPCs with a 40ft movement speed can break Action Economy, whereas a Beast with an Int of 3 can only follow instructions to the limits of its comprehension.
That said, I'm obviously a fan of Blink Dogs. Individually, they may not be very impressive, but being able to conjure eight of them turns them into something unto themselves.
So, that aside, the Killmoulis(CR 0) has a cute ability that you might want to add to your list.
Blessing of Bountiful Generosity (1/day) Targets 8 creatures. The next time a target finishes a long rest, it regains all spent Hit Dice and gains 10 temporary hit points.
Since it triggers after a Long Rest, the effect last for 24 hours and is basically a freebie if you have the slot to spare. (If you are preparing to protect a town, you could boost 64 villagers per casting.)
Boggle(CR 1/8) aren't very strong, but they also have two abilities of note:
Oil Puddle [Slippery/Sticky] - Lasts for an hour and creates an arbitrary number of traps that can cause restrained or prone. (Great for setting up an ambush.)
Dimensional Rift - Can create a portal to anywhere within 30ft of itself as a bonus action. (Since it does not require line of sight, it can literally see through walls and interact with anything within range, as a bonus action. Major Utility.)
Since "Eight fey creatures of challenge rating 1/4 or lower" doesn't specify that they all have to be the same, a clever player could conjure a Mite to grant disadvantage on the Boggle's Oil Puddle, and then have 5 Blink Dogs attacking the prone target with advantage (Fey are immune to the Mite's Vexing Presence). Tossing in a Killmolis to get a good night's rest. Any time left with the Boggle and Blink Dogs in a dungeon-type environment becomes gravy.
I don't have much experience using this spell, and I want to explore some options aside from the cheesy pixie-polymorph combo. I'll be rating them on a 5 star system.
CR 1/4, 8 creatures summoned.
Blink Dogs **
their damage is pretty bad. They have a teleport. Maybe you could do something cool with using them for mounts if you have a party full of goblins, gnomes, or other small creatures that can ride medium creatures. Overall, not very useful.
Sprites *****
Now, these are kind of interesting. Their HP and damage are abysmal, BUT they use poisoned arrows that cause Poisoned for 1 minute on a DC 10 CON save and Unconscious on a DC 6 CON save. They also have a fly speed of 40 and can turn invisible at will (+8 stealth). I think you could do some really creative things with sprites. If you have an archer in your group, you could have them share their poisoned arrows (obviously subject to DM approval). On their own, they could still do some cool stuff such as help you silently infiltrate a camp. Even with +3 con, there's a 73% chance that at least ONE of those arrows will cause the mob to fall unconscious.
Sprites also have an ability where they can determine a creature's alignment. Maybe situationally useful.
Mite *****
Similar to Sprites, Mites have terrible HP and damage (better than sprites though), but have a really interesting trick up their sleeve. Mites can place a hex on a creature. The hex has a recharge of 6 and there is no save. It can be done while hidden without revealing the mite's location. The hex is quite powerful: "While the creature is affected by this hex, whenever it makes an attack roll, an ability check, or a saving throw, it must roll a d6 and subtract the number rolled from the d20 roll". The fact that you can summon 8 of these things and hex a whole group of enemies is pretty incredible. The hex has a strange stipulation that when it rolls the d6, the creature can attack an ally to ignore the effects of the hex until the end of its next turn.
CR 1/2, 4 creatures summoned.
Satyr *
The only thing these offer is damage. You'd be better off with conjure animals.
Darkling **
These are mildly interesting. They have a feature similar to sneak-attack that requires advantage. If you can set that up, their damage isn't that bad (1d4+2d6+3). Light sensitivity means they are limited in daylight though. When killed, they let off a bang that can blind enemies for 1 turn (DC 10 con save).
CR 1, 2 creatures summoned.
Dryad *****
Dryads are pretty dope. They are spellcasters. Each one gets 3 casts of Entangle and Goodberry as well as 1 cast of Pass Without Trace. You can get 60 points of healing from the goodberries (will take a few minutes for people to much them down of course), which about matches what you could get from a 4th level cast of Healing Spirit. Dryads are perfect for summoning prior to sneaking off somewhere. You can get covered by PWT for free, get your 60 goodberries, then the Dryads can attempt to Entangle enemies when you start combat.
Dryads also have a few other tricks up their sleeves. They can speak with beasts and plants, so you might be able to get some information from nearby trees. They have Tree Stride, making them extremely mobile in a forest. And they also have a Fey Charm ability that allows them to charm 1 humanoid or up to 3 beasts. Imagine being in a forest. You summon your dryads. Then you send them off to gather beasts to help you out. They each return with 3 wolves under their charms. Nice, right?
CR 1, 2 creatures summoned.
Nereid****
This creature has some pretty nice abilities. Drowning Kiss (Recharge 5-6) is a con save with nice acid damage and renders the target unable to speak on a fail. Could be very useful for targeting enemy casters. Blinding Acid is an attack that... can blind a target on a hit. It can also cast Control Water, including using it to create a whirlpool for up to a 30 ft cube of water.
Naiad**
This creature is only really useful for its spellcasting which includes a single cast of fly and hypnotic pattern as well as 3 casts of phantasmal force. Not that great, but it's a roundabout way to get access to those spells which might be useful in the right situation.
Note: By design, the DM chooses what actually appears, the player only chooses the number/CR. [SageAdvice]
Something to consider about Blink Dogs:
Their Teleport action includes a free bite attack, so they can run in as a mob, get 8 attacks with advantage (flanking), and then teleport 40ft out of melee/Aoe range without provoking Attacks of Opportunity.
With a 1 hour duration, you might want to take advantage of 8x perception checks with advantage. These are intelligent creatures who can look for traps, treasure, and communicate important information non-verbally. They can also fetch and bring things of value back to you.
Properly run, Blink dogs can be very effective. Just depends on where you are and what you need.
True, it's ultimately up the the DM what creatures you summon. I feel like most good dm's will consider your request and at the very least not give you completely useless summons. Here is the relevant part of that Sage Advice answer
For blink dogs, IMO they compare pretty poorly with wolves. Conjure Animals can summon the same number of wolves which do about twice as much damage and have pack tactics (a lot of games don't do flanking). Blink Dogs do have better HP and the blink ability. They aren't useless, but they aren't good either IMO
A Shark compares pretty poorly to a Rat, if you compare it by how quickly it can run on land. :)
In a typical encounter, creature actions are often simplified to a fairly finite set of behaviors. It's true that if limited to mob behavior, the wolf will probably do more damage over the course of 6 rounds. However, if played in a dynamic world with clever players, the Blink Dog is effectively an NPC, rather than an animal. It has human levels of intelligence, understands two languages, and can take initiative if given latitude ("Kill them" or "help us", as opposed to "bite them").
A wolf might bite for 2d4+2 damage, but a Blink Dog could teleport onto a ledge and knock over a statue/boulder to deal massive damage or change the battlefield.
A wolf might get advantage via Pack Tactics, but a Blink Dog could trigger traps, snuff out lights, and understand resistances and vulnerabilities.
If your party were locked in a cell, you could tell the Blink Dogs "Get Help", and they would understand that getting help could either mean finding an ally, or tracking down a guard with keys and dragging them back to you. When they return, they might volunteer information that you didn't think to ask for, such as the number of guards and the fastest way to the outside.
Having 8 teleporting NPCs with a 40ft movement speed can break Action Economy, whereas a Beast with an Int of 3 can only follow instructions to the limits of its comprehension.
That said, I'm obviously a fan of Blink Dogs. Individually, they may not be very impressive, but being able to conjure eight of them turns them into something unto themselves.
So, that aside, the Killmoulis(CR 0) has a cute ability that you might want to add to your list.
Blessing of Bountiful Generosity (1/day) Targets 8 creatures. The next time a target finishes a long rest, it regains all spent Hit Dice and gains 10 temporary hit points.
Since it triggers after a Long Rest, the effect last for 24 hours and is basically a freebie if you have the slot to spare. (If you are preparing to protect a town, you could boost 64 villagers per casting.)
Boggle(CR 1/8) aren't very strong, but they also have two abilities of note:
Oil Puddle [Slippery/Sticky] - Lasts for an hour and creates an arbitrary number of traps that can cause restrained or prone. (Great for setting up an ambush.)
Dimensional Rift - Can create a portal to anywhere within 30ft of itself as a bonus action. (Since it does not require line of sight, it can literally see through walls and interact with anything within range, as a bonus action. Major Utility.)
Since "Eight fey creatures of challenge rating 1/4 or lower" doesn't specify that they all have to be the same, a clever player could conjure a Mite to grant disadvantage on the Boggle's Oil Puddle, and then have 5 Blink Dogs attacking the prone target with advantage (Fey are immune to the Mite's Vexing Presence). Tossing in a Killmolis to get a good night's rest. Any time left with the Boggle and Blink Dogs in a dungeon-type environment becomes gravy.