King Mswati III has urged Swaziland's politicians not to engage in ritual killings to boost their chances in October's elections.
Mswati is Africa's only absolute monarch
The BBC's Tom Holloway in the capital, Mbabane, says that the number of ritual murders increases at election time.
There is a strong belief that magic charms are more powerful if they contain human body parts.
The king was speaking after unveiling a new draft constitution at the weekend, which did not meet the demand of democracy activists for multi-party elections in Africa's last absolute monarchy.
During election times, we tend to lose our grandmothers, grandfathers and young children. They just disappear
However our correspondent says the constitution does include the rights to the freedom of expression and association, so parties may be able to meet and campaign, even if they are not allowed to field candidates.
Swaziland has been without a constitution for the past 30 years.
"During election times, we tend to lose our grandmothers, grandfathers and young children. They just disappear. But I want to warn you all that you should not resort to ritual murder," King Mswati III said in a televised address.
Elections have been held regularly despite the ban on politcal parties, with voters choosing between independent candidates, our correspondent says.
The king also dissolved parliament until the elections but he said this did not mean he would rule Swaziland alone; he would be assisted by a council of ministers.
Obed Dlamini, a former prime minister and head of a banned political party, the Ngwani National Liberatory Congress, told the BBC he was disappointed with the continuing ban on political parties but said the new constitution - the first since 1973 - was a step forward.
He said he thought most of Swaziland's population believed the king should have absolute power.