I have a question, if you have a level 6 wizard, and that person was able to temporarily learn say a level 8 or 9 spell, maybe through a special ritual or something, but, that person does not have a spell slot for  that high a level spell, could that person still cast that spell?

I was thinking maybe through the use of a special crystal or some other special object that would allow that person to cast a high level spell even though they have no spell slot for it.

You're basically describing a magic item. Yes, it's possible for a magic item to let a spellcaster cast a spell of a level that's normally too high for them.

A magic item, feat, class feature, or racial trait can all allow a character to cast spells without needing slots.

You could also just "it happens because narrative" it.

Ok, I think that is what I will do, I'll give her a trait that would allow it. I want her to be able to manipulate time and she needed the "time stop" spell to do it.

A "trait" would make it part of her character, whether it be class or race. I would say a one use item, or maybe an item that needs special conditions to be met in order for it to be used.

What is the intent here? Is it part of the story that this Character be able to stop time once - in which case a one use item would be best.

If the intent is that this Character be able to stop time repeatedly - then be very cautious about game balance if this a Player Character. DMThac0's suggestion as to make specific ( and presumably very limited ) conditions which this ability could be used is a good one - otherwise you know Players will find ways to use this ability in ways you hadn't expected or planned for.

If this an NPC - then you don't even need to consider mechanics at all. It's really just as simple as "this NPC needs to be able to do this, at this point in the Narrative", so ... they can and do! No need to try and find a game system way to justify it. It's just magic! ;) And of course, they'll never use it outside of those Narrative requirements ( or it won't work, for some odd reason ).

Spell scrolls exist for that exact purpose. If you have a spell scroll that contains a spell from your class' spell list and it's of a higher level than you can normally cast you can try casting it with a DC 10+Spell level Arcana check. On a success you cast the spell and the scroll becomes nonmagical, and on a failure the spell is wasted and the scroll becomes nonmagical.

If you wanted to change that, you could make the scroll a guaranteed success, or reusable, or both.

Well the character is an NPC but the way she is being used in story is as a normal player, the ability to manipulate time is needed for the story, she is a warforged being, geneticly built by magic, she is good, and only uses her abilitities for the good of all. She can only use this ability once after a long rest.

Quote from Princexylan478 >>

Well the character is an NPC but the way she is being used in story is as a normal player, the ability to manipulate time is needed for the story, she is a warforged being, geneticly built by magic, she is good, and only uses her abilitities for the good of all. She can only use this ability once after a long rest.

 Oh, so it is a DMPC that has the ability to cast a free level 9 spell once per day. That is a whole new can of worms.

Just try not to make your players feel like side characters.

Quote from DxJxC >>

Quote from Princexylan478 >>

Well the character is an NPC but the way she is being used in story is as a normal player, the ability to manipulate time is needed for the story, she is a warforged being, geneticly built by magic, she is good, and only uses her abilitities for the good of all. She can only use this ability once after a long rest.

 Oh, so it is a DMPC that has the ability to cast a free level 9 spell once per day. That is a whole new can of worms.

Just try not to make your players feel like side characters.

It could potentially be a force multiplier for the party, though.  Can she only stop time for herself, or can she make the whole party act during that time?  Make her unable to do anything while sustaining the effect, and give the party a free round while she's holding it.

Quote from Nkeyo >>

Spell scrolls exist for that exact purpose. If you have a spell scroll that contains a spell from your class' spell list and it's of a higher level than you can normally cast you can try casting it with a DC 10+Spell level Arcana check. On a success you cast the spell and the scroll becomes nonmagical, and on a failure the spell is wasted and the scroll becomes nonmagical.

If you wanted to change that, you could make the scroll a guaranteed success, or reusable, or both.

I was going to reply this. Use the Arcana check to add a layer of difficulty in casting something normally beyond the wizard's abilities.

Quote from Princexylan478 >>

Well the character is an NPC but the way she is being used in story is as a normal player, the ability to manipulate time is needed for the story, she is a warforged being, genetically built by magic, she is good, and only uses her abilities for the good of all. She can only use this ability once after a long rest.

She can use this ability once per long rest:  that's all the rules justification that you need to worry about right there. She can just do this thing; full stop. 

As pointed out elsewhere, be very clear about what her ability does: who is time-stopped, for how long, what else ( if anything ) can she do that turn, how long does it last, etc.

This NPC was actually created by a PC who no longer plays D&D (health issues) . She filled me in on all of her abilities, and how they work, she used to use her in a Doctor Who campaign a while back. She can only stop time once per long rest (Aprox. 12 hours). Also, she must have plane shifted into one of her two existing planes (accelerated time, or Decellerated Time) in order to do it. It does stop existing time for everyone in a 20 mile radius , the people inside the affected area is not aware of what is happening, the effect only lasts for 2 minutes (her time) she can only manipulate current events within that 2 minute period. Once the two minutes are up, time starts back up normally for everyone. Now any manipulation she did if in accelerated time plane, will happen instantly, but if she was in decellerated time plane when the manipulation was done, there will be a 2 minute delay for everyone in normal time. There is some sort of temporal displacement in decellerated time but no one is aware of it. When she returns to normal time, she will see the effects of her manipulation when it occurs. She can only shift to one of these planes once a day and can only remain in them for a short period (forgot the timing on that, don't have her character sheet handy) . The only reason I was asking some insight on this is because I've never played any sort of ...I guess you could call her a "Time Lord" of such in D&D, she did a wonderful job of changing her to fit into a D&D campaign. I just wish the actual player was around to play her, that would make things interesting. Thank the gods she's good and not evil.

Yes but there is a chance of failure which I dont remember. But basically they have to make an arcana check AMD if they succeed they cast the spell. The idea being that the spell scroll not only is a manual for the spell but also is charged with the energy needed to cast said spell.