What is the ruling on engraving ḥarqūs on the back and stomach of the bride for adornment for her husband?

A woman adorning herself for her husband with henna (harqous) designs is permissible, as long as it is not a permanent tattoo, and provided two conditions are met: First, the design must not be on the area between the navel and the knee, as this is 'awrah (intimate area) that may not be exposed except to the husband. Second, the design must be removed if it prevents water from reaching the skin during ablution (wudu') or ritual bath (ghusl (definition)). If it washes off with water, then there is no harm in it.