This will be an allegation of forgery of the signature of the testator.
When it is shown that the testator's name has been forged to the will in question, the will is void.
Burden of Proof
After execution of the will is proven, the burden of proof is on the caveator to show that the signature of the testator is forged.
You can attack the validity of the testator's signature is by the use of handwriting experts, but this in the face of direct credible evidence will not alone invalidate.