Runo Stromkirk // Krothuss, Lord of the Deep — ruling 1

You choose which opponent or opposing planeswalker the token is attacking as you put it onto the battlefield. It doesn't have to be the same player or planeswalker Krothuss, Lord of the Deep is attacking.

Runo Stromkirk // Krothuss, Lord of the Deep — ruling 2

Although the token is attacking, it was never declared as an attacking creature (for purposes of abilities that trigger whenever a creature attacks, for example, like training).

Runo Stromkirk // Krothuss, Lord of the Deep — ruling 3

The token copies exactly what was printed on the original creature and nothing else (unless that permanent is copying something else or is a token; see below). It doesn't copy whether that creature has any counters on it or Auras and/or Equipment attached to it, or any non-copy effects that changed its power, toughness, types, color, and so on. Notably, it doesn't copy any effects that may have turned a noncreature permanent into a creature. If the token isn't a creature as it enters the battlefield, it won't be attacking.

Runo Stromkirk // Krothuss, Lord of the Deep — ruling 4

If the copied creature had {X} in its mana cost, X is 0.

Runo Stromkirk // Krothuss, Lord of the Deep — ruling 5

If the copied creature was copying something else, the token enters the battlefield as whatever that creature was copying.

Runo Stromkirk // Krothuss, Lord of the Deep — ruling 6

If the copied creature is a token, the token created by Krothuss copies the original characteristics of that token as stated by the effect that put it onto the battlefield.

Runo Stromkirk // Krothuss, Lord of the Deep — ruling 7

Any enters-the-battlefield abilities of the copied creature will trigger when the token enters the battlefield. Any “As [this creature] enters the battlefield” or “[This creature] enters the battlefield with” abilities of the copied creature will also work.

Runo Stromkirk // Krothuss, Lord of the Deep : règles, fonctionnement et interactions (MTG)

Runo Stromkirk // Krothuss, Lord of the Deep 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

Flying When Runo enters, put up to one target creature card from your graveyard on top of your library. At the beginning of your upkeep, look at the top card of your library. You may reveal that card. If a creature card with mana value 6 or greater is revealed this way, transform Runo. Flying Whenever Krothuss attacks, create a tapped and attacking token that's a copy of another target attacking creature. If that creature is a Kraken, Leviathan, Octopus, or Serpent, create two of those tokens instead.

How it works : Runo Stromkirk // Krothuss, Lord of the Deep

Original English text — Wizards of the Coast / Scryfall

#1November 19, 2021· Wizards of the Coast
You choose which opponent or opposing planeswalker the token is attacking as you put it onto the battlefield. It doesn't have to be the same player or planeswalker Krothuss, Lord of the Deep is attacking.
No other card is named in this ruling.
#2November 19, 2021· Wizards of the Coast
Although the token is attacking, it was never declared as an attacking creature (for purposes of abilities that trigger whenever a creature attacks, for example, like training).
No other card is named in this ruling.
#3November 19, 2021· Wizards of the Coast
The token copies exactly what was printed on the original creature and nothing else (unless that permanent is copying something else or is a token; see below). It doesn't copy whether that creature has any counters on it or Auras and/or Equipment attached to it, or any non-copy effects that changed its power, toughness, types, color, and so on. Notably, it doesn't copy any effects that may have turned a noncreature permanent into a creature. If the token isn't a creature as it enters the battlefield, it won't be attacking.
Card named in this ruling
Copy
#4November 19, 2021· Wizards of the Coast
If the copied creature had in its mana cost, X is 0.
No other card is named in this ruling.
#5November 19, 2021· Wizards of the Coast
If the copied creature was copying something else, the token enters the battlefield as whatever that creature was copying.
Card named in this ruling
Copy
#6November 19, 2021· Wizards of the Coast
If the copied creature is a token, the token created by Krothuss copies the original characteristics of that token as stated by the effect that put it onto the battlefield.
No other card is named in this ruling.
#7November 19, 2021· Wizards of the Coast
Any enters-the-battlefield abilities of the copied creature will trigger when the token enters the battlefield. Any “As [this creature] enters the battlefield” or “[This creature] enters the battlefield with” abilities of the copied creature will also work.
No other card is named in this ruling.