What is meant by "the conclusion of deeds," and can underestimating a certain action after performing many good deeds lead to entering Hellfire, as in the saying: "If one does many good deeds and then commits a sin, and then their soul is taken, their destiny will be to Hellfire, and refuge is sought with Allah"?

The outcomes of deeds determine a person's destiny. Whoever's life ends in goodness is among the people of Paradise, and whoever's life ends in evil is among the people of Hellfire. If the end is disbelief, he will abide eternally in Hellfire. As for sin, whoever repents from it will be forgiven. If he does not repent, and his good deeds outweigh his bad deeds, then he is among the people of Paradise. But if his bad deeds outweigh his good deeds, then he is under the will of Allah.