The ornamental design for a sceptre finial, as shown.
FIG. 1 shows a perspective view of a sceptre finial incorporating the design;
FIG. 2 shows a front elevational view of the sceptre finial shown in FIG. 1, the rear elevational view, top plan view, and bottom plan view being identical;
FIG. 3 shows a right side elevational view of the sceptre finial shown in FIG. 1; and,
FIG. 4 shows a left side elevational view of the sceptre finial shown in FIG. 1.