dnd 5th – Summon high CR monsters with Ally Planar

Yes.

But you, the clerk, you can not choose what will happen. By the text of Planar Ally:

This entity sends a celestial, elemental, or faithful demon to help you, making the creature appear in an unoccupied space within reach. If you know the name of a specific creature, you can pronounce that name when you cast that spell to ask that creature, though could get a different creature anyway (Choice of DM).

There is no limitation to the CR of the creature, of any nature whatsoever. The DM will most likely try to fit the scenario and your relationship with the entity of another word. Note that the creature summoned …

is under no constraint behave in a particular way. You can have the creature perform a service in exchange for a payment, but do not have to do this.

… although it is sent mainly help you:

This entity sends a celestial, an elemental or a demon loyal to it help you

Nevertheless, you should pray your DM right after your cleric finishes praying for a planar ally.

dnd 5th – This homebrew race is Mind Flayer balanced?

I've found a homebrew breed Mind Flayer * and I wonder if it is reasonably well balanced compared to other breeds.

I've removed the stuffed animals like age and so on. and I just left the important parts.

My personal conclusion is that it could be on the strong side, but stay within the limits of power.

Increased capacity score

Your intelligence score increases by 2, and your charisma score
increases by 1.

Cut.

Most illithids are taller than humans, but their bodies are thin,
weighing much less than a being average of their stature. Your size
is average.

Speed.

Your basic walking speed is 30 feet.

Darkvision.

You have darkvision up to a range of 60 ft.

Magic resistance.

You have an advantage over saving throws against spells and other magics
effects.

Mental attack.

You can use your action to attack anything in front of you with a spirit
explosion. When you use your mental breath, every creature in a 15-foot cone
must make an intelligence saving throw equal to 8 + your intelligence
modifier + your skill bonus.

A creature suffers 2d6 psychic damage during a missed save and half less
damage on a successful backup. This damage increases to 3d6 to 6
level, 4d6 at the 11th level and 5d6 at the 16th level. After using your
mind blowing, you can not use it again until you fill a short or long
rest.

Devour the brain.

If you consume the brain of a recently deceased creature, you
interface with his mind. In doing so, you learn at least one piece of
useful information that the creature knew.

Telepathy.

You can talk telepathically to any creature within 120 feet of you. A
The creature understands you only if you share a language.

Languages.

You can speak, read and write Common and Undercommon, and can read
and write Qualith.

* I do not remember where I found that, so apologize to the creator to whom I can not give credit.

dnd 5th – Can you cast the Elemental Conjure spell to summon an elemental in the Forbiddance spell's effect area?

the prohibition spell description states:

You create a protection against magical travels that protects up to 40,000 square feet of floor space at a height of 30 feet above the ground. For the duration, creatures can not teleport to the area or use portals […] enter the area. The spell also protects the area from planar movement and thus prevents creatures from accessing it through the astral, ethereal, Feywild, Shadowfell, or change of plane to spell.

In addition, the spell damages the creature types you choose when you cast it. Choose one or more of the following choices: celestial, elemental, fecund, demon, and undead. When a chosen creature enters the area of ​​the spell or begins its turn, the creature suffers 5d10 radiant or necrotic damage (your choice when you cast that spell).

When you cast this spell, you can designate a password. A creature that pronounces the password on entering does not suffer any damage with the spell.

The area of ​​the spell can not overlap prohibition to spell. If you cast this spell every day for 30 days at the same place, the spell lasts until it dissolves, and the hardware components are consumed in the last cast.

Can you throw the conjure elementary spell to summon an elemental in the area of ​​effect of the prohibition to spell?

5th dnd – Is it possible to increase the AC of a mirror image duplication?

The spell clearly states:

The AC of a duplicate is equal to 10 + your dexterity modifier.

This is very explicit. D & D works according to the "Specific Beats General" rule (PHB p.7), which means that in this case, you can modify the formula if you find a feature that explicitly modifies the AC of a Mirror image illusion, or usually changed the AC from any illusion created by the caster.

However, to my knowledge, no official rule book contains a feature or ability that would have such an effect. The place most likely to find such a feature would be the class of a illusionary magician, but they are disappointed in this regard.

Therefore, the CA will still be calculated as $ left (10 + DEX right) $and since 10 is obviously constant, the only possibility is to increase your DEX modifier. You can do this, for example, with the UPSs obtained during the upgrade, the equipment of items increasing your DEX statistic, or the use of magical items that continually increase your DEX.

Examples of such articles would be a Ioun Stone (increases DEX by 2, up to 20, when worn) or Manual of speed of action (continuously increases DEX and DEX maximum score by 2).