To select all the lines starting from STARTING_PATTERN up to  blank line ^$ and then delete those lines.

# sed ‘/STARTING_PATTERN/,/^$/d’ filename

To edit files in place, use -i option.

# sed -i ‘/STARTING_PATTER/,/^$/d’ filename

Insert multiple lines into a file after specified pattern.

# sed ‘/cdef/r add.txt’ input.txt

# sed ‘/cdef/r add.txt’ input.txt

input.txt: abcd accd cdef line web	add.txt: line1 line2 line3 line4
Output : abcd accd cdef line1 line2 line3 line4 line web

If you want to apply the changes in input.txt file. Then, use -i with sed.

# sed -i ‘/cdef/r add.txt’ input.txt

If you want to use a regex as an expression you have to use the -E tag with sed.

# sed -E ‘/RegexPattern/r add.txt’ input.txt