The verse "He commanded his servants to fill the grave" from Tobit 8:18 refers to Tobit, after his son Tobias and daughter-in-law Sarah safely returned home and after Sarah's previous misfortunes were revealed.

Here's a breakdown of what it means in context:

Context of Tobit 8: This chapter describes the wedding night of Tobias and Sarah. Sarah had previously been married seven times, but each husband had been killed by the demon Asmodeus before the marriage could be consummated. Through the guidance of the angel Raphael (disguised as a human companion), Tobias drives away the demon and consummates the marriage.
Significance of the Grave: In the earlier part of chapter 6, Tobit, concerned for his son's safety, prepared a grave for him. The assumption was that Tobias would likely die like Sarah's other husbands. Now that Tobias has safely returned, the grave is no longer needed for its intended purpose.
"He commanded his servants to fill the grave": Tobit's command to fill the grave symbolizes several things:
Relief and Joy: It's a physical manifestation of Tobit's overwhelming relief and joy that his son is alive and well. The grave is no longer a symbol of mourning and expected death, but a reminder of the miraculous events that transpired.
Faith and Divine Intervention: Filling the grave acknowledges the role of divine intervention in Tobias's survival. They can now put the expectation of death to rest.
New Beginnings: It marks a new beginning for the family. The threat of death has passed, and they can now look forward to a future filled with hope and happiness.
Disposal of Preparations: It is a very practical matter. Since the grave had been readied, it was a hole in the ground that would need to be filled in for sanitation and safety reasons.

In short, "He commanded his servants to fill the grave" is a powerful image that signifies the triumph over adversity, the gratitude for divine protection, and the beginning of a new chapter in Tobit's family's life.