Cruelty is an old common law ground for divorce which does not exist under the Matrimonial Causes Act but can be used to establish that the respondent has behaved in ways that the petitioner can no longer be reasonably expected to live with him or her as provided in section 15(2).
Cruelty is the intentional an malicious infliction of physical and mental harm and suffering or a reasonable fear of further harm. It also includes the continuous infliction of minor acts of ill treatment that are likely to cause the victim to break down emotionally or mentally. However, temper and nagging are not enough to establish cruelty.
Strong evidence of cruelty includes police reports, medical records and eye witness testimony are desirable but not indispensable. The court is required to consider the entire evidence and decide whether it gives rise to a finding of cruelty. Generally conduct that endangers life, limb or health will amount to cruelty.
This will include persistent, pervasive and excessive violence or threat of violence, deliberately infecting a spouse with a sexually transmitted disease, constant displays of rage, use and abuse of juju/voodoo or religious doctrine neglect of a spouses physical needs for food, shelter and clothing and verbal and emotional abuse.
It should be noted that under the act the requirement is to prove that the petitioner/victim can no longer be reasonably expected to live with the respondent and the burden of proof is significantly lighter than it was to prove cruelty under common law.
The Nigerian case law on section 15(2) is remarkably well developed. Make sure your lawyer is knowledgeable about them and can cite the appropriate cases.