How to to handle abstract base class in entity framework(code first)?

I am implementing an audit functionality to keep track of any changes (create, update, delete) made to any type of object. For this I need a way to declare a generic object which can point to object of any other class derived from an abstract base class.

Base class I do not want to have table for in database :

  public abstract class EntityBase {
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    [DataType(DataType.Date)]
    public DateTime CreationDateTime { get; set; }

    [DatabaseGenerated(DatabaseGeneratedOption.Computed)]
    [DataType(DataType.Date)]
    public DateTime ModificationDateTime { get; set; }
  }

  public class EntityBaseConfigurations<TEntity> : IEntityTypeConfiguration<TEntity> where TEntity : EntityBase {
    public virtual void Configure(EntityTypeBuilder<TEntity> builder) {
      builder.Property(e => e.CreationDateTime).IsRequired().HasDefaultValueSql("GETUTCDATE()");
      builder.Property(e => e.ModificationDateTime).IsRequired().HasDefaultValueSql("GETUTCDATE()");
    }
  }

and a couple of models derived from base for example :

  [Table("Initiative")]
  public class Initiative : EntityBase {
    [Key]
    public int InitiativeId { get; set; }

    // ...

    [Required]
    public Contributor Assignee { get; set; }
  }

  public class InitiativeConfiguration : EntityBaseConfigurations<Initiative> {
    public override void Configure(EntityTypeBuilder<Initiative> builder) => base.Configure(builder);

    // ...
  }

  [Table("Contributor")]
  public class Contributor : EntityBase {
    [Key]
    public int ContributorId { get; set; }

    // ...
  }

  public class ContributorConfiguration : EntityBaseConfigurations<Contributor> {
    public override void Configure(EntityTypeBuilder<Contributor> builder) => base.Configure(builder);

    // ...
  }

Now my failed attempt to create the Audit model

  [Table("Audit")]
  public class Audit : EntityBase {
    [Key]
    public int AuditId { get; set; }

    public User Actor {get; set;}

    // ...

    public EntityBase Entity { get; set; } // I want to be able to point to objct of any class derived from EntityBase (ex. Initiative, Contributor)
  }

  public class AuditConfiguration : EntityBaseConfigurations<Audit> {
    public override void Configure(EntityTypeBuilder<Audit> builder) => base.Configure(builder);
    // ...
  }

When I try to create a migration for audit class, I get below error

The derived type 'Audit' cannot have KeyAttribute on property 'AuditId' since primary key can only be declared on the root type.

Here is my Db Context if needed

  public class DbContext : IdentityDbContext<User> {
    public DbContext(DbContextOptions<DbContext> options) : base(options) { }

    public DbSet<Initiative> Initiatives { get; set; }
    public DbSet<Contributor> Contributors { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder) {
      base.OnModelCreating(modelBuilder);
      modelBuilder.ApplyConfiguration(new InitiativeConfiguration());
      modelBuilder.ApplyConfiguration(new ContributorConfiguration());
    }
  }