Casting long-casting-time spells undetected

Question

The objective is to cast a long casting time spells like Tiny Hut or Private Sanctum that can give you a battlefield advantage.

For example Tiny Hut allows your team to have a position from which you and your allies can shoot ranged weapons without being counterattacked by spells or weapons, Private Sanctum can deprive opponents that can teleport from doing so, or create a visibility shield around a fortification with siege weapons you plan to attack with a larger force.

Normally the casting time prevents the use of these spells in combat. Assume you know where the opponents are. The challenge is to cast the spell close enough to them so the spell will help in combat, without being detected before you are done casting.

Ideal would be solutions that work with just the core classes, feats or spells from the PHB, do not need to rely on optional rules, and that also do not require access to specific magic items. (Although other solutions are also OK).

Is there a way to do this?

Answer 1

Try your hardest to be a Chronurgy Wizard.

The Chronurgy Wizard published in Explorer’s Guide to Wildemount is probably the easiest way to do what you’re trying to do. I discussed this in my answer to this question: Can Arcane Abeyance allow you to cast a spell longer than one action in one action?. The Arcane Abeyance feature states:

When you cast a spell using a spell slot of 4th level or lower, you can condense the spell’s magic into a mote. The spell is frozen in time at the moment of casting and held within a gray bead for 1 hour. […]

A creature holding the bead can use its action to release the spell within, whereupon the bead disappears. The spell uses your spell attack bonus and save DC, and the spell treats the creature who released it as the caster for all other purposes.

This feature allows the Wizard to create a quick release tiny hut, or my favorite spell, Galdur’s tower grenade, as the spells come into effect immediately upon releasing them from the mote.

As long as you can be somewhere safe to cast the spell, you then have an hour to let it loose from the mote.

Answer 2

I know it's a lame answer, but it should be there so:

Cast Wish

It has a casting time of 1 Action, and whatever spell (of 8th level or lower) you duplicate just happens; so it gets around long casting times.

This also doesn't run a risk of losing access to it, since duplicating spells is the basic usage.

Answer 3

Subtle Spell can help with remaining undetected.

I assuming your intention is to begin casting before combat starts, then stealthily approach while casting, so that you complete the casting when combat starts. In this case, the sorcerer's subtle spell metamagic option is probably the most straightforward way of remaining stealthy while you cast.

Answer 4

You say:

You know where the opponents are. The difficulty is to get close enough to them so the spell will help in combat, and to cast the spell before they detect you.

So you would like to close on the enemy, then cast tiny hut for example, as a way to gain a tactical advantage.

I think your spells are examples of preparing favorable terrain. Sure, you could use spells, but digging trenches or throwing up walls would have the same general effect.

Tiny hut is problematic because it takes so long, so it will only work in limited situations.

Don't take the favorable terrain to the enemy, bring the enemy to the favorable terrain

Prep your terrain, then lure the enemy.

This could be as simple as retreating until you reach your favorably prepared terrain.

This can be ideal because you can conceivably have significant time to prepare, from a minute to cast tiny hut to much longer for more elaborate preparations.

Limitations: you need to know ahead of time when and where the battle will be; you need to lure the enemy.

Use illusion

You might be able to use illusion to disguise what's going on or create a distraction.

Major image could give you an obscuring illusion, such as of a herd of cattle or similar, to obscure the sight and sound of your caster. Alternatively, you might be able to create an illusion to focus their attention elsewhere. A "dragon" attacking might give you a round or two to prepare terrain.

Hallucinatory terrain might be able to confuse the enemy enough that while they may hear the caster, they can't see them.

Limitations: Costs spells, you may not have these spells available. Maybe the enemy will catch on.

Use distraction

Get someone else (either in the party or not) to attack them from the other side. Maybe they'll the too busy to attack you.

Limitations: Requires the cooperation of someone else. And maybe they won't be too busy to attack you.

Time stop

Time stop is made for this. You get 2-5 turns to prep your terrain.

Limitations: 9th level spell, and if it were available to you, maybe you have better options. Roll badly and you just burned your slot for a measly 2 rounds of prep.

Conclusion

No tactic will work in every situation, and this is no different. But these might be situationally useful.

Answer 5

Use a bag of holding

Have an ally put the caster in a bag of holding. Have the caster immediately start casting. The ally needs to keep track of the time and, just before the spell is ready to be deployed, pull the caster out of the bag.

Answer 6

Use Stealth, Hiding and Pass without Trace

It may be a bit boring, but this is a simple approach that does not require high-level spells or special equipment: use Dexterity (Stealth) checks to sneak close to the enemies and cast the spell quitely from a hidden position. If someone in your group can cast pass without trace, this can make this much easier: it is muffling sound and the +10 bonus on stealth checks is a big boost.

You also can use invisibility to get there if the area you need to pass to approach is under surveillance, or you may be able to get there with teleportation like dimension door. Once you start casting, you will become visible if you used invisibility, so you need to be out of sight by then.

There is no fixed limit on the required volume of spellcasting. It is clear that it normally would be noticeable, and you need to at least succeed in a Dexterity (Stealth) check to whisper and avoid being heard. In very quiet circumstances, it may be impossible to do so, but with +10 from Pass without Trace, and proficiency, you have a very good chance to succeed even on a hard check.