It appears you might be looking for a word similar to "henceforth" in Welsh. The most common translation for "henceforth" in Welsh would be "o hyn ymlaen".
Here are some other ways to express "henceforth" in Welsh:
* "o'r awr hon ymlaen": From this hour onwards
* "erbyn hynny ymlaen": From then onwards
* "am byth": Forever
It's important to choose the most appropriate translation depending on the context of your sentence.