What would be more effective: Immunity to nonmagical or general physical resistance?

Question

I'm home brewing a Tarrasque-like monster for a high level one shot I'm planning, and for a mix of lore and gameplay reasons I'm debating if my beast should have the typical "immune to Bludgeoning, Piercing, and slashing damage from nonmagical weapons" or straight up just resistance to those types of attacks, magical or not.

One on hand the immunity makes sense since this is supposed to have been a beast so powerful entire kingdoms had to put their best warriors and mages together to stop it, not just send an army of horse archers to kite it to death.

On the other hand the heroes of this one shot definitely already have magic weapons and attacks, so this really only affects summons and transformations the mages might do, emphasis on might because one is an evocation wizard who loves their pew pews. So the immunity will more or less just be ignored

The players have been asking for a challenge and I find the base Tarrasque a shadow of its former self, but I'm not sure if the general resistance might be considered too annoying considering it still does have like 656 HP with regeneration on top of that.

Answer 1

The monster should continue to have immunity, for the reason you described -- otherwise it is too vulnerable to low-level archers.

If you want it to be harder to kill using physical weapons, consider just giving it more hit points.

Answer 2

Give it more hit points and immunity

According to the DMG, p. 277, for self-created monsters

If a monster has resistance or immunity to several damage types-especially bludgeoning, piercing, and slashing damage from nonmagical weapons-and not all the characters in the party possess the means to counteract that resistance or immunity, you need to take these defenses into account when comparing your monster's hit points to its expected challenge rating. Using the Effective Hit Points Based on Resistances and Immunities table, apply the appropriate multiplier to the monster's hit points to determine its effective hit points for the purpose of gauging its final challenge rating.

Expected CR	HP Multiplier for Resistances	HP Multiplier for Immunities
17 or more	x1	x 1.25

In your case, all characters have ways to deal magical damage, so there is no reason to take the immunity to normal weapons into account when constructing the challenge. The characters would not have a way around "resistance to all damage", but the DMG also recommends:

Assign a vulnerability, resistance, or immunity to a monster only when it's intuitive. For example, it makes sense for a monster made of molten lava to have immunity to fire damage. Giving a monster resistances and immunities to three or more damage types (especially bludgeoning, piercing, and slashing damage) is like giving it extra hit points.

As DanB recommends the best solution here is to just give it more hit points instead of resistance to all damage.

You can throw in immunity to nonmagical physical damage for free for narrative reasons (i.e. why an army of mooks does not work against it), without affecting your CR either.

One note in additon: on this high level of play, you need ways to protect the monster from something like a forcecage which can take it out without saves if it is not too large or can teleport in some way. You likewise will need other protective measures like legendary resistance, or all the hp in the world will not save it from being transformed into a chair by true polymorph and put into a demiplane to sit there for eternity.