Fblthp, the Lost — ruling 1

There's normally no way to cast Fblthp from your library or to have it enter the battlefield from your library. You'll have to use other effects to find Fblthp for the bonus card draw.

Fblthp, the Lost — ruling 2

If an effect exiles Fblthp from your library and then lets you cast that card, it's cast from exile, not from your library.

Fblthp, the Lost — ruling 3

Fblthp's last ability triggers only if it's on the battlefield when it becomes the target of a spell.

Fblthp, the Lost — ruling 4

Fblthp's last ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.

Fblthp, the Lost — ruling 5

If the spell that targets Fblthp has no other targets, it won't resolve (because it no longer has a legal target after Fblthp has gotten totally lost in your library).

Fblthp, the Lost : règles, fonctionnement et interactions (MTG)

Fblthp, the Lost is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

When Fblthp enters, draw a card. If it entered from your library or was cast from your library, draw two cards instead. When Fblthp becomes the target of a spell, shuffle Fblthp into its owner's library.

How it works : Fblthp, the Lost

Original English text — Wizards of the Coast / Scryfall

#1March 19, 2021· Wizards of the Coast
There's normally no way to cast Fblthp from your library or to have it enter the battlefield from your library. You'll have to use other effects to find Fblthp for the bonus card draw.
No other card is named in this ruling.
#2March 19, 2021· Wizards of the Coast
If an effect exiles Fblthp from your library and then lets you cast that card, it's cast from exile, not from your library.
No other card is named in this ruling.
#3March 19, 2021· Wizards of the Coast
Fblthp's last ability triggers only if it's on the battlefield when it becomes the target of a spell.
No other card is named in this ruling.
#4March 19, 2021· Wizards of the Coast
Fblthp's last ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.
No other card is named in this ruling.
#5March 19, 2021· Wizards of the Coast
If the spell that targets Fblthp has no other targets, it won't resolve (because it no longer has a legal target after Fblthp has gotten totally lost in your library).
No other card is named in this ruling.

Interactions with other cards

These cards' text mentions Fblthp, the Lost.

Case of the Lost Witness

When this Case enters, conjure four cards named Fblthp, the Lost into your library, then shuffle. Draw a card. To solve — You control a legendary Homunculus. Solved — You may look at the top card of your library any time and you may play lands and cast spells from the top of your library.