Q: Concerning the Servants' appearance, do they take the form they had just before death, or the form they had during the prime of their lives?

A: Generally speaking, they take the appearance they had in their prime. However, if there's some kind of strong curse, or the Servant in question has an attachment to it, then their form from just before death can also appear.

Comptiq 2007 Issue 10; Fate Dojo Q&A