Roedd Jason Cooper wedi bod o flaen llys o'r blaen ar gyhuddiad o geisio llofruddio Laura Jayne Stuart, ond clywodd y llys ddydd Gwener bod Ms Stuart wedi marw.
Ymddangosodd Mr Cooper, 27 oed o Ddinbych, ar gyswllt fideo o garchar Altcourse yn Lerpwl.
Mae'n wynebu cyhuddiadau o lofruddio Ms Stuart yn Ninbych ar 12 Awst, ac o glwyfo David Roberts gyda'r bwriad o achosi niwed corfforol difrifol iddo.
Doedd dim cais am fechnïaeth ac fe gafodd y diffynnydd ei gadw yn y ddalfa nes i'r achos yn ei erbyn ddechrau ym mis Chwefror, 2018.
Er bod dyn wedi ei gyhuddo, mae Heddlu Gogledd Cymru yn pwysleisio fod eu hymchwiliad yn parhau, a bod swyddogion yn dal i chwilio am y gyllell gafodd ei defnyddio i drywanu Ms Stuart.
Mae'r heddlu yn gofyn i unrhyw un sydd â gwybodaeth i gysylltu drwy ffonio 101 neu 0800 555 111 gan ddefnyddio'r cyfeirnod RC 1712 2068.

Unicode, the authority on computer text and characters, has already approved adding the three flags to the next release of emojis in June.
Technology companies such as Apple, Google, Microsoft and Facebook will be able to introduce the flags to their software from July.
A UK flag is already available on smartphones and other devices.
Unicode develops the mechanism for making the flags available, but manufacturers have to add the new emojis to their keyboards.
Emoji flags already available include those for Ascension Island, St Barthelemy, Curacao, Diego Garcia and Djibouti.
The campaign for a new range of emoji flags was started by Jeremy Burge of Emojipedia and BBC Wales' social media lead Owen Williams in March 2016.

Aberdeenshire-based Harbro Group saw pre-tax profit increase by £800,000, to £3.5m, in the year to the end of June 2016.
This was despite turnover falling by £1m to just over £100m.
Harbro said international sales grew over the year, both in Europe and in new markets further afield.
The Turriff-based company manufactures and supplies feeds for the beef, sheep, dairy, pig and poultry sectors.
It also has a network of 19 shops across Scotland, operating under the Country Store brand. The stores cater for farming and rural communities with a range of products, including fireside supplies, country clothing and footwear.
In accounts filed with Companies House, the company said: "As always, the outlook for agriculture is difficult to predict.
"We have certainly been experiencing challenging times over the last 12-18 months but the industry is a robust one.
"The impact of Brexit is still to be fully understood, although the board are confident that the group will be well positioned for whatever business challenges this may bring about."
Mae dyn wedi ymddangos o flaen Llys y Goron Yr Wyddgrug i wynebu cyhuddiad o lofruddiaeth.
[NEXT_CONCEPT]
Emoji flags for Wales, Scotland and England could be available on tablets and smartphones by the summer.
[NEXT_CONCEPT]
One of Scotland's largest animal feed producers has posted a rise in profits, despite reporting a "challenging" year for the industry.