Another 'inner ___' calque. Japanese doesn't build phrases this way.

What a Japanese person actually thinks
Reads like the chapter heading of a translated business book. Five characters of nothing much, two of them hiragana.

Meaning

Literally: inner + strength
Actually: Understandable, but 内なる is a literary construction rarely used outside translated Western self-help. The whole phrase is visibly imported.

Scores

Naturalness●●●●●

The construction exists but is almost entirely confined to translationese.

Coolness●●●●●

Long, mixed script, no visual density. It looks like a caption rather than a tattoo.

Watch out

Nothing embarrassing lurking in here.

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剛毅 gōki
fortitude; firmness of character

Two characters, real word, exactly this meaning.
不動心 fudōshin
an immovable mind

Strength as something that cannot be shaken. Established term in martial arts, and far more striking.

On skin

内なる強さ

Orientation: either

N/A — change the words.