Damage at 0 Hit Points.
If you take any damage while you have 0 hit points, you suffer a death saving throw failure. If the damage is from a critical hit, you suffer two failures instead.
Beginning at 9th level, you can reroll a saving throw that you fail. If you do so, you must use the new roll, and you can't use this feature again until you finish a long rest.
Beginning at 14th level, your mastery of ki grants you proficiency in all saving throws.
Additionally, whenever you make a saving throw and fail, you can spend 1 ki point to reroll it and take the second result.
Can you reroll the automatic fail death saves with these (and/or others)? I kinda think Indomitable works, but I'm less sure that "suffering a death save failure" counts as "making" the save for Diamond Soul.