dnd 5e – Can the spells of a Flameskull be counteracted?

Counterspell requires the following:

[a reaction], which you take when you see a creature within 60 feet of you casting a spell

The Flameskull's spell casting block reads as follows:

[The Flameskull] does not require somatic components or materials to cast its spells.

If the Flameskull does not require visible components, can its spells be counteracted?

dnd 5e – Flameskull resistant to magic piercing damage?

Most creatures in D & D 5e have details about what attacks or types of weapon damage are resistant. For example

Resistance to damage: Aggress, pierce and graze from non-magical attacks

However, the description of Flameskull carries resistance to drilling damange without the phrase "From Nonmagical Weapons / Attacks":

Resistance to damage: lightning, necrotic, piercing

Flameskull, Manual of Monsters, p. 134

Does not that mean that the Flameskull is resistant to magic piercing damage as well as non-magic piercing damage?

dnd 5e – Can you win endless XP using a Flameskull and its auto-revival feature?

Yes, but …

It's not that big of an agreement. As NautArch points out,

With respect to Combat XP, the DMG simply says:

Each monster has an XP value based on its level of challenge. When the adventurers defeat one or more monsters, usually killing them, defeating them or capturing them, divide the total XP value of the monsters evenly among them.

Technically, you're just killing him, let him reform and then kill him again … but did you really kill him?

If the DM determines that this is a valid "XP factory" and that killing it is the same as defeating it, you will have to understand the cost of that factory.

A Flameskull has a challenge rating of 4 and is worth 1,100 XP. Assuming you have a group of 4 players in level 4, each time you kill the Skull, each of you gains 275 XP. 275 XP per hour at the cost of fighting a creature that can cast a large number of spells an unlimited number of times is not much. With the minimum minimum of 2,700 XP required to be level 4, it would take 14 hours and 14 encounters with this skull to reach level 5 and another 28 hours and 28 encounters to reach level 6.

Think of the cost of fighting a creature with 40 HP, 13 BC, with multiple resistances, immunities and condition immunities. It's not as easy as killing him instantly, because if you can not kill him instantly, he can throw a fireball every time he resuscitated. Or, it can flee, attack or, otherwise, lengthen the encounter to become more deadly.

In general, the characters are not destined to have 14 medium challenge matches a day and are not equipped to fight so many times without resting a lot. It is not a great feat, it is more a strategy with a great cost. And if he wants to continue fighting, he can never stop to rest a lot or leave him alone for more than an hour because, in all probability, he will flee or do something to stop his difficult strategy.)

This does not say anything about whether your DM really allows you to do this in a vacuum or do it at all. But yes, if you want to make a very slow XP grinder, you can do it.

dnd 5e – In the Lost Mine of Phandelver adventure, when does Flameskull attack?

Note: I am French, and although I am fluent in English, I am sometimes baffled by a phrase.

In P. 46, under room 12 ("Smelter Cavern"), he says that Flameskull's orders are to prevent anyone from passing through the room he defends.

Does that mean that Flameskull only attacks if you try to cross the room?

If someone takes a couple of steps and then back down the same corridor they entered due to the zombies, would that trigger the flameskull or not?

dnd 5e – In The Mine of Phandelver, when does Flameskull attack?

Note: I am French, and although I am fluent in English, I am sometimes baffled by a phrase.

It is said that the flameskull's orders are to prevent someone from passing through the room that he defends.

Does that mean that Flameskull only attacks if you try to cross the room? If someone takes a couple of steps and then back down the same corridor they entered due to the zombies, would that trigger the flameskull or not?