Get or create a costume "Staff of the Planes"
I couldn't find such an item in the rules, but the rules for staves creation allow creating new types of staves. It will be extremely expensive, but you'll get an item which will allow you to use Greater Create Demiplane up to 10 times before needing to find a more powerful caster to recharge it for you (until you reach a level allowing you to cast 9th level spells, of course - then you could recharge it yourself).
Also, keep in mind that most of the unique benefits of Greater Create Demiplane over it's "standard" and "lesser" are something you may use sparingly - you can create a functioning demiplane using the "standard" and "lesser" versions, and use the "greater" variant only for adding a permanent portal or change the way time work there etc. (which, since the "greater" variant affects more space of a plane, you will need less castings to cover your entire demiplane)
If you really have enough money, I'd get 2 different staves:
- A "Staff of the Plains" which I'd need help recharging but grants access to Greater Create Demiplane.
- A "Minor Staff of the Plains" which doesn't have the Greater Create Demiplane power, but which I'd be able to charge on my own - this one will be used for most of the plane creation / modifications I'd need.
Thematically, I think the "Staff of the Plains" should have something along the following powers:
- Minor Create Demiplane (1 charge)
- Create Demiplane (2 charges, or 1 charge if you pay extra during creation)
- Greater Create Demiplane (3 charges, or 1 charge if you pay extra during creation)
Maybe also some helpful powers for a demiplane creator / keeper:
- Plane Shift (maybe limited to travel between a created demiplane and the Material, Astral and Ethereal planes)
- Permanency (limited to effects on the created demiplane)
- Some other niceties such as Tiny Hut or Magnificent Mansion etc.
If you want me to add the costs for such staves, or more information on how to create / charge them, please indicate that in the comments...
At any rate, this is something you'll need to work out the details with your DM - he may require you to research the creation of such an item and create it yourself, rather than simply allow you to commission such an item from a really powerful wizard...
Yes; the ruling is now official and explicit from Sage Advice
As Dale M's previously-accepted answer said, the PHB rules for the Arcane Trickster imply that its class spell list is the Wizard's spell list.
This was made explicit in the official Sage Advice, which says:
For you to meet a spell scroll’s requirement, the spell on the scroll needs to be on whatever spell list is used by your class. Here are two examples. If you’re a cleric, the spell must be on the cleric spell list, and if you’re a fighter with the Eldritch Knight archetype, the spell must be on the wizard spell list, because that is the spell list used by your class.
From the example of a Eldritch Knight, we know that there is no class list of spells for the fighter class, but the class list for a fighter that becomes an Eldritch Knight is the wizard class list. We can conclude that there is no class spell list for rogues, but a rogue that becomes an Arcane Trickster has the wizard spell list, "because that is the spell list used by your class."
Thus an Arcane Trickster can use spell scrolls of spells that are on the Wizard spell list.
Thanks to InternetHobo who encouraged Rubiksmoose to cite the SA Compendium in their answer.
Best Answer
An Arcane Trickster 3/Conjurer 2 knows the following:
From Conjurer
Three cantrips from the Wizard list
Six 1st-level Wizard spells (gained at level 1)
Two 1st-level Wizard spells (gained at level 2)
Anything he's found scrolls for.
Note that because you consider the classes individually when determining spells known, the character could not select 2nd-level spells at level 2, even though the character may have the slots (as described below).
The character can prepare (Conjurer Level)+(INT Mod) spells from the Wizard list as a Conjurer.
From Arcane Trickster
Mage Hand plus two cantrips from the Wizard list.
Three 1st-level Wizard spells, two of which must be enchantment or illusion.
All of the spells gained from Arcane Trickster are considered prepared at all times.
The only part of multi-classing where the two classes have anything to do with each other is in determining spell slots, as quoted above. In the case of an Arcane Trickster 3/Conjurer 2, the character has the spell slots of a third level caster, because (3/3)+2=3. This means the character has four 1st-level and two 2nd-level slots.
If the character has a spell that can be cast in a higher level slot for added effect, the character can do so. Even spells without an added benefit can be cast from a higher level slot, they just don't do any more than they would out of the slot they normally use. Either way, the character doesn't know any 2nd-level spells despite having slots for them.