meanings:

1. mystery - something that is difficult or impossible to understand or explain.

2. portal - a doorway, gate, or other entrance, especially a large and imposing one.

3. river - a large natural stream of water flowing in a channel to the sea, a lake, or another such stream.

4. lonely - isolated, sad.

5. attack - an aggressive and violent action against a person or place.

sentences:

1. It is a complete mystery what caused the accident.

2. He waited for the portal behind her to close.

3. They swam across the river.

4. She thinks about him whenever she’s feeling lonely.

5. People are attacking the mayor for breaking campaign promises.