who 己 is indicating in this sentence

Question

The person the speaker is talking to previously warned of disaster if the the speaker didn't do something, but the speaker did not want to break traditions and did not interfere. That person shows up again in front of the speaker after that disaster did in fact occur.

other person 私の話を思い出して、ここに……魔を封印した場所に来たのですね？

speaker: ここに来たのは、己が揺らがぬよう戒めるためです。

speaker その他に意味などあり得ません。

other person いい加減に分かってください！人間に地上界を治める資格などありません！

You came here to admonish me for being steadfast (to my beliefs).

• ここに来たのは、己(me)が揺らがない様（を）戒めるためです。

You came here to admonish me in a resolute way.

• ここに来たのは、己(you)が揺らがない様（に）戒めるためです。

Normally it's pretty obvious from context who 己 is referring to, but i found it difficult in this case.

thank you

Answer 1

Judging from the context you provided, I can be pretty sure that it means myself.

己{おのれ} could surely sometimes mean oneself and sometimes you, but their registers are so different that we almost never confuse them in real settings. 己 means you in vulgar talks (or common in some dialect), and oneself in elevated parlance.

In this case, it is clear that speaker is giving a formal, bookish line, not only in form but in matter. Thus it is only natural to be interpreted as oneself, in this case, myself.

PS
Not knowing the plot, I'm not sure whether the 己 means "belief" as in your translation, or "spirit", "soul", "super-ego" kind of thing (opposed to some demon or evil).

PPS
The translation of the second line:

ここに来たのは、己が揺らがぬよう戒めるためです。

should be literally like:

The reason I came here is to restrain myself lest it loosen/waver.

The accurate interpretation of 己, 揺らぐ, and 戒める is up to the context, though.