Imagine how terrifying this would be. Your friend just says “I’m going to ask an ancient being a few questions, give me a minute” and then loses their sanity.

War Mage makes light work of this spell as early as level 10.
with 2 feats/ASIs or even just Headband of Intellect, any wizard could easily be at LEAST 18 for a +4 mod by level 9
Every Wizard is proficient in Int saving throws, Proficiency is +4 at level 9
When War Mage is concentrating on ANY spell (even Skywrite) Saving throws get +2 at level 10
If a War Mage fails a saving throw, they can use a reaction to give their Saving throws +2 at level 2
The Lowest you can roll on a d20 is a 1
4+4+2+1=11 +4 reaction=15

Now.... there is a small catch. TECHNICALLY when the casting time of a spell is longer than when fits in a single turn, you have to maintain concentration on that spell, which will interrupt other concentration spells you might have going. It's not ABSOLUTELY clear if this also applies to spells that have a natural casting time of a single turn, but have that time extended via casting it as a ritual.
Theoretically if you cast C.O.P. as a ritual, it's using concentration, so you can't concentrate on Skywrite for the +2 bonus, BUT at that point does concentrating on casting a spell count as maintaining that spell? It's not really maintained if you fail in the middle of casting it. If so, a War Mage wouldn't even NEED to cast Skywrite in order to get the Concentration Bonus, just casting C.O.P. as a ritual would activate the +2.

Of course, it's all a moot point as early as level 13. At that point you have a +5 to prof, and if you get your Int up to 20, you're right back at the 15 minimum even if you don't have the concentration bonus. If you don't max your int, you have to wait until level 17 and 18 int to be immune.

It's interesting that a War Mage can be better than even a Divination specialist at casting one of the most powerful divination rituals.

Can you use this spell to talk to your dead friend whose body was disintegrated? It would actually talk to their spirit right?

Or be a divination wizard and use a portent.

Cast Intellect Fortress before this :P 

D&D developers are terrible at determining how long something should take. Duration of 1 minute means you only have 12 seconds to ask each question and receive a response.

Quote from andrewjo >>

D&D developers are terrible at determining how long something should take. Duration of 1 minute means you only have 12 seconds to ask each question and receive a response.

Wibbly wobbly, timey wimey....

Nothing says that the person casting the spell perceives time in the same way while concentrating. While it may appear to be 1 minute for everyone else, it could be 10 for the caster. 

Quote from Wolfblood013 >>

Quote from andrewjo >>

D&D developers are terrible at determining how long something should take. Duration of 1 minute means you only have 12 seconds to ask each question and receive a response.

Wibbly wobbly, timey wimey....

Nothing says that the person casting the spell perceives time in the same way while concentrating. While it may appear to be 1 minute for everyone else, it could be 10 for the caster. 

 Could also be that asking each is instant, the minute is just a limit on how much stuff you can let happen after casting that might change what you ask or what the answers are before the spell simply runs out.

I can see that. Not to mention, it doesn't say much about how the questions are asked. I interpret this as the questions and answers are dealt in the matter of the speed of thought, and occurs within the mind: that's why the caster could take psychic damage and go insane, right?

Quote from Bruised_Battler >>

I can see that. Not to mention, it doesn't say much about how the questions are asked. I interpret this as the questions and answers are dealt in the matter of the speed of thought, and occurs within the mind: that's why the caster could take psychic damage and go insane, right?

 Presumeably the Psychic Damage reason is because not all contacts appreciate being called on and also the caster might just screw up.

Or perhaps they cannot handle the forces that they commune with.

A great instance of this spell being used is on The Dungeon Run is episode 83. The party warlock uses it to speak with the "Old Ones," And the DM is very creative in his answering.

I would rule it as you have one minute to ask a question and then it resets the timer to ask another question until you had all your questions answered

Quote from Akamoria >>

Cast Intellect Fortress before this :P 

 I don't know if you are allowed to do that.  I think casting a spell with a casting time longer than 1 action costs your concentration.  intellect fortress requires concentration.  

Quote from EnchantedNinja12345 >>

Quote from Akamoria >>

Cast Intellect Fortress before this :P 

 I don't know if you are allowed to do that.  I think casting a spell with a casting time longer than 1 action costs your concentration.  intellect fortress requires concentration.  

 Someone else can cast it on you

Quote from GreyAcumen >>

War Mage makes light work of this spell as early as level 10.
with 2 feats/ASIs or even just Headband of Intellect, any wizard could easily be at LEAST 18 for a +4 mod by level 9
Every Wizard is proficient in Int saving throws, Proficiency is +4 at level 9
When War Mage is concentrating on ANY spell (even Skywrite) Saving throws get +2 at level 10
If a War Mage fails a saving throw, they can use a reaction to give their Saving throws +2 at level 2
The Lowest you can roll on a d20 is a 1
4+4+2+1=11 +4 reaction=15

Now.... there is a small catch. TECHNICALLY when the casting time of a spell is longer than when fits in a single turn, you have to maintain concentration on that spell, which will interrupt other concentration spells you might have going. It's not ABSOLUTELY clear if this also applies to spells that have a natural casting time of a single turn, but have that time extended via casting it as a ritual.
Theoretically if you cast C.O.P. as a ritual, it's using concentration, so you can't concentrate on Skywrite for the +2 bonus, BUT at that point does concentrating on casting a spell count as maintaining that spell? It's not really maintained if you fail in the middle of casting it. If so, a War Mage wouldn't even NEED to cast Skywrite in order to get the Concentration Bonus, just casting C.O.P. as a ritual would activate the +2.

Of course, it's all a moot point as early as level 13. At that point you have a +5 to prof, and if you get your Int up to 20, you're right back at the 15 minimum even if you don't have the concentration bonus. If you don't max your int, you have to wait until level 17 and 18 int to be immune.

It's interesting that a War Mage can be better than even a Divination specialist at casting one of the most powerful divination rituals.

 Correction:  War Mages can use their reaction to gain a +4 bonus to a failed saving throw.  You wrote +2.  The +2 bonus is for AC.  It's even better than you expected.  

Quote from JShepherd1 >>

Can you use this spell to talk to your dead friend whose body was disintegrated? It would actually talk to their spirit right?

 The catch:  "long-dead sage."  Therefore your friend must not only be full of knowledge and wise, they must be ancient.  

Quote from GreyAcumen >>

War Mage makes light work of this spell as early as level 10.
with 2 feats/ASIs or even just Headband of Intellect, any wizard could easily be at LEAST 18 for a +4 mod by level 9
Every Wizard is proficient in Int saving throws, Proficiency is +4 at level 9
When War Mage is concentrating on ANY spell (even Skywrite) Saving throws get +2 at level 10
If a War Mage fails a saving throw, they can use a reaction to give their Saving throws +2 at level 2
The Lowest you can roll on a d20 is a 1
4+4+2+1=11 +4 reaction=15

Now.... there is a small catch. TECHNICALLY when the casting time of a spell is longer than when fits in a single turn, you have to maintain concentration on that spell, which will interrupt other concentration spells you might have going. It's not ABSOLUTELY clear if this also applies to spells that have a natural casting time of a single turn, but have that time extended via casting it as a ritual.
Theoretically if you cast C.O.P. as a ritual, it's using concentration, so you can't concentrate on Skywrite for the +2 bonus, BUT at that point does concentrating on casting a spell count as maintaining that spell? It's not really maintained if you fail in the middle of casting it. If so, a War Mage wouldn't even NEED to cast Skywrite in order to get the Concentration Bonus, just casting C.O.P. as a ritual would activate the +2.

Of course, it's all a moot point as early as level 13. At that point you have a +5 to prof, and if you get your Int up to 20, you're right back at the 15 minimum even if you don't have the concentration bonus. If you don't max your int, you have to wait until level 17 and 18 int to be immune.

It's interesting that a War Mage can be better than even a Divination specialist at casting one of the most powerful divination rituals.

I appreciate the math but you would be hard pressed to find an 8th+ level Wizard that would struggle to consistently pass a DC 15 Int save enough to go through so much trouble.

Concentration only ends if the new spell you cast requires concentration, so Akamoria’s plan would work

Quote from EnchantedNinja12345 >>

Quote from Akamoria >>

Cast Intellect Fortress before this :P 

 I don't know if you are allowed to do that.  I think casting a spell with a casting time longer than 1 action costs your concentration.  intellect fortress requires concentration.  

 Concentration only ends if the new spell you cast requires concentration 

Quote from MonksAreGreat70 >>

Quote from EnchantedNinja12345 >>

Quote from Akamoria >>

Cast Intellect Fortress before this :P 

 I don't know if you are allowed to do that.  I think casting a spell with a casting time longer than 1 action costs your concentration.  intellect fortress requires concentration.  

 Concentration only ends if the new spell you cast requires concentration 

 Per the article Don't Lose Focus! Your Guide to the Rules for Concentration in D&D that can be found on this site:

There are two additional sources of concentration, which tend to be easily forgotten:

• First, any spell with a casting time longer than one action requires concentration for the duration of its casting time. So while mending, prayer of healing, animate dead, and similar spells are not concentration spells per se, they do require concentration during their actual casting process. This also applies to spells being cast as rituals, as the casting time of such spells is more than an action. If concentration is lost, the spell slot is not expended, but if you still want that spell to be cast, you’ll have to start over from the beginning. 
• Second, using an action to ready a spell requires concentration until that spell is cast, even if the spell isn’t usually a concentration spell.

Source: https://www.dndbeyond.com/posts/1224-dont-lose-focus-your-guide-to-the-rules-for

Which is to say, a single caster cannot maintain Intellect Fortress and still cast Contact Other Plane, because casting Contact Other Plane takes longer than the duration of a single action and thus requires concentration.