Fusion is a special technique that is known only to Witches.
Fusion is a survival technique that consists of merging one's life force with others. However, after assuming a new shape under the Fusion, a Witch cannot return to his/her previous appearance (unlike Transformation Magic).
This technique also allows the user to absorb other beings and, by merging with them, allows the user of the technique to become even more powerful. This was seen why Lady Oyakata was draining Inner Moka and when she "ate" Ruby.
However, this technique does have one additional drawback; the user's magical attribute. If the Witch's magical attribute (a Wand or Spell Book) is destroyed, then this technique will fail. And, failing, it will cause an enormous explosion.
It does seem, however, that the Witch has a few moments left before they die from the explosion. This is seen when Lady Oyakata mulls over what she had done before she used the last of her energy to separate herself from Ruby, thus saving Ruby's life.
The explosion following the failure of Fusion while enormous, is not large enough to destroy a large city. As an example, when Oyakata's Fusion failed, the explosion decimated Oyakata and the Hanabake plants she melded to, but the blast didn't destroy the city Oyakata had sought to burn.
Due to Fusion's drawbacks, it can be classified as "Severely-High Risk" or as an extreme last resort move. Judging from Ruby's distress upon seeing Lady Oyakata use this technique, it is not a choice even a very desperate Witch would make.