What are the last words of Horatio in the play? (396-400) What does he mean and what must he do from now on?

The last words from Horatio are as follows:

HORATIO:

Not from his mouth,
Had it the ability of life to thank you.
He never gave commandment for their death.
But since, so jump upon this bloody question,(390)
You from the Polack wars, and you from England,
Are here arrived, give order that these bodies
High on a stage be placed to the view;
And let me speak to the yet unknowing world
How these things came about. So shall you hear(395)
Of carnal, bloody and unnatural acts;
Of accidental judgments, casual slaughters;
Of deaths put on by cunning and forced cause;
And, in this upshot, purposes mistook
Fall'n on the inventors' heads. All this can I(400)
Truly deliver.

HORATIO:

Of that I shall have also cause to speak,
And from his mouth whose voice will draw on more.
But let this same be presently perform'd,
Even while men's minds are wild, lest more mischance(410)
On plots and errors happen.

He is explaining that it will be his duty to retell the tragic events that have transpired and to clarify the errors of judgement made to allow the catalogue of slaughter that remains on the stage at the end of the play. Horatio’s role is to encourage the audience to reflect on what they have seen, and as the play ends, the audience itself has to establish ‘How these things came about’. It is Horatio's role to highlight that the tragedy has come about through ‘mistaken motives’ – that Hamlet was unable to testify to his own procrastination as the means by which the King could work to destroy all that he loved. Horatio is asked by Hamlet as his dying wish in Act V Scene II to reveal the truth -

Horatio, I am dead.
You live. Tell the story of me and my cause correctly
To those who are not satisfied.