The same scriptures in the Amplified Bible, regarding Joshua 24, as follows:
And Joshua wrote these words in the book of the law of God; and he took a great stone, and set it up there under an oak that was in [the court of] the sanctuary of the Lord. Certainly nothing druid here
And Joshua said to all the people, See, this stone shall be a witness against us; for it has heard all the words the Lord spoke to us; so it shall be a witness against you, lest [afterward] you lie[pretend] and deny your God.
; in fact they just got through killing off the pagan thirty-one Cannanite Kings and conquering their cities. It took them about six years to do all this.
Now they were finalizing their mutual association,
"vs. 28 So, Joshua sent the people away, evey man to his inheritance."
They now had their promise land.
The stone [boulder] was a symbolic marker [a witness against them] for future reference to observe. It was to be a reminder that the tribes of Israel were to serve only the God of Israel and not to serve other strange gods.
This should be a reminder today for those who involve themselves in pagan religions and spiritism, to be warned and know how God feels about their serving other Gods.