Limitations of Illusory Reality
Illusory Reality states:
By 14th level, you have learned the secret of weaving shadow magic into your illusions to give them a semireality. When you cast an illusion spell of 1st level or higher, you can choose one inanimate, nonmagical object that is part of the illusion and make that object real. You can do this on your turn as a bonus action while the spell is ongoing. The object remains real for 1 minute. For example, you can create an illusion of a bridge over a chasm and then make it real long enough for your allies to cross.
The object can't deal damage or otherwise directly harm anyone.
In short, the Wizard can manifest an object which:
- is real, inanimate, and non-magical
- cannot directly harm anyone
- disappears after one minute
Moreover, because the object manifested by the wizard is real, it follows all the normal rules for object detailed in the DMG's Objects section (p. 246-247). In particular:
For the purpose of these rules, an object is a discrete, inanimate item like a window, door, sword, book, table, chair, or stone, not a building or a vehicle that is composed of many other objects.
This limits the complexity of what can be considered an object and excludes indiscrete things like gasses, liquids, and holes.
Finally, the Wizard cannot use Illusory Reality to disappear or transform a preexisting objects and the Wizard has no special control over the manifested object.
Curtail Illusory Reality by
Preventing its use
Anything that prevents the Wizard from casting spells, also shuts down Illusory Reality because the Wizard needs to cast an Illusion spell to create an object. Some relatively common examples which fall in this category and which specifically hinder casters are silence and counterspell.
Dispel magic can also dispel an illusion and prevent the use of Illusory Reality, but do note that dispelling the illusion after the Wizard has manifested an object will not cause the object to disappear.
Rolling for uncertain outcomes
The Wizard can manifest an object without fail within the limits of Illusory Reality, but that does not mean that the Wizard always achieves the desired outcome.
The DM should not try to undermine the player, but when an outcome is uncertain, the DM can (and often should) call for a roll, be it an ability check or a save. Referencing traps and spells can help to understand when a roll is adequate.
For example, the Wizard can create an adamantine dome using Illusory Reality, but whether that dome will capture a creature as it manifests is uncertain. The DM could rule that the creature is just captured like force cage or call for a save like wall of stone. Both are equally valid, though I would urge for a consistent choice on the DM's part.
Destroying the object
Once the object exists, the characters can destroy it. The Objects section mentioned previously offers some guidelines:
When characters need to saw through ropes, shatter a window, or smash a vampire's coffin, the only hard and fast rule is this: given enough time and the right tools, characters can destroy any destructible object. Use common sense when determining a character's success at damaging an object. Can a fighter cut through a section of a stone wall with a sword? No, the sword is likely to break before the wall does.
Of course, the objects only exists for one minute, so a thick wall or boulder is probably going to survive that minute no matter what the DM throws at it, but that is not the case for all objects.
The same section of the DMG includes tables to determine the AC and HP of objects. For example, an adamantine cart is a large object with 23 AC and 27 HP. Enemies suitable for a party of level 14 characters can easily destroy something like that in one round.
I recommend reading the whole Objects section in preparation for a Wizard with Illusory Reality.